Mystery Noise after stone passed under car

[edgeshift]

Hi All,

On a recent journey a truck dumped a load of stones in front of my car. There was little I could do to avoid them and although I missed the larger stones a couple went under the car. They were not large, but large enough for them to bank and scrap. As soon as they had worked there way free the car immediately started to make a banging/knocking noise. I pulled over but there was no obvious damage. I set off again but the banging/tapping was constant, increasing with intensity the faster I went. As I was not far from home I drove home in second gear with my hazards on and parked up.

I took the car to Murphy & Gunn a BMW main dealer in Dublin. They serviced the car as it was due anyway and I asked them to look at the problem. To my amazement they said they couldn't find anything, they could clearly here the noise but could see no damage. There only suggestion was that it might be a wheel bearing or the differential, the cost of replacement €4000. Given the car is running fine aside from the noise, this was unlikely, if either of these went at the exact moment a stone went under the car it would be quite a coincidence. They could suggest nothing else which left me annoyed and exasperated but there was little I could do aside from take the car back home.

Previously I had been happy with the garage however there lack of interest or help has put me off them and I couldn't recommend them any more.

I took a look at the car myself at the weekend, just lifting it on the jack and spotted what looks like the path the stone took see photos below, highlighted areas in red. These shots were taken from the front of the car to the back in a sequence.











The noise starts at about 5kmph as a constant banging/knocking. As I accelerate it increases in intensity becoming very rapid, as I decelerate it slows down. It sounds like its coming from the rear drivers side of the car but it is hard to tell exactly. It sounds like something either flapping against something as air passes under the car or something caught in something that rotates and hits something on each revolution.I have recorded the noise but don't know how to add this to the post?

The other aspect of the noise is that as you accelerate rapidly it disappears for a short time before returning.

Any help in identifying what this could be would be most appreciated.

Regards,

Paul Z3 2002 2.2i Sport

[Del]

Only a wild guess - a piece of exhaust heat shield bent/loose/damaged and knocking against a rotating propshaft?

Edit - or even a stone lodged between the heat shied and rotating propshaft?

[edgeshift]

Thanks Del,

I take it I wont be able to get at this with the regular jack that came with the car.

Is there a schematic of the underside of a Z3 locating the propshaft so I can try and identify the problem with a camera?

Paul

[pingu]

I would get the front of the car up in the air and properly supported on axle stands. Rotate the front wheels to check for noises.

Put the front wheels on ramps and support the rear wheels on axle stands under the lower arms (*).

I would normally say to remove the undertrays, but you don't have any. I suggest you get some.

Rotate the rear wheels, listening for suspicious noises.

Using a rubber mallet, tap everything in sight under the car.

With engine off and car in neutral, turn the propshaft and listen for rattles. I can see that you have some access at the rear of the gearbox (be careful, the exhaust may be hot).

Start the engine and run the car in gear. You can go through the gears upto about 30mph (**). You can go as fast as you like, but there's little point.

If you are on your own, being careful, you can leave the car idling in 1st and get under the car to check. The wheels will be turning at about 5mph.



(*) This is so you can run the car without damaging the driveshafts.
(**) If you run the car faster than 30mph, the speedo will stop working. Don't panic, this is because the front wheels aren't turning. It will sort itself out when you next drive with all four wheels on the ground.

[BladeRunner919]

I take it I wont be able to get at this with the regular jack that came with the car.

That jack is only for wheel changes. Using it for any maintenance is likely to end in you getting hurt. If you get under the car using that jack you're liable to end up dead!

[Mike Fishwick]

Yes - buy a trolley jack and get some large wooden blocks . . . if you value your life!

Whatever - check at the tyres for a stone caught in the tread, and also check the brakes for a stone trapped between the water shield and the edge of the disc, and being thrown about as the wheel rotated.

[peter2b]

if the sound increases with speed its got to be some thing touching a rotating item,prop shaft,half shaft ,brakes dink in dif cover stone in side wall on wheel
+ second the tip DONT ever get under a car with just a jack ,a chap up the road from me an experienced mechanic did just that, car slipped off jack onto his chest ,he was found 4 hours later dead
happy days
peter2b

[gookah]

a chap up the road from me an experienced mechanic did just that, car slipped off jack onto his chest ,he was found 4 hours later dead
happy days

Did he overcharge you at some point?

[Boosh]

I bought a spare wheel today, as didn't have any. I found a large chunk of tarmac in the spare wheel tray! Could you have 'gained' a stone, lodged somewhere and rattling?
Boosh.

[Mugs]

i had a similar thing last year when i drove on a nicely re-'stoned' country road.
i managed to track it down to a stone stuck in the near side caliper which sounds like it shouldn't make a banging noise but it did, almost like a hammer tapping once every wheel revolution. this obviously got more frequent and louder the faster i went.
worth a check.

[therealdb1]

When you do eventually find the culprit perhaps you should take a photo and take it back to the stealer and ask why they did not find the problem that they were expressly asked to look for. My guess is that they just did the routine service and left it at that as the message to look for the particular issue never made it past the receptionist!
Incidentally I have a real industrial trolley jack that would lift a bus but I still wouldn't crawl under a car supported only by this. Axle stands or better still railway sleepers under the wheels if they are staying fitted during the work should always be used.
A jack is for lifting not supporting!

[edgeshift]

Thanks to everybody for their suggestions, I'll take another look this weekend, but won't get under the car unless its appropriately supported.

Anybody know how to upload a sound file to the forum, this may help identify it easier, I have recorded the sound at different speeds.

The most likely culprit seems to be a bent heat shield knocking against the propshaft or a fragment of the stone in the brake caliper.

Any other ideas welcome.

When I finally discover the problem I'll report back.

[Robert T]

Anybody know how to upload a sound file to the forum, this may help identify it easier, I have recorded the sound at different speeds.

What format did you record in? You might be able to upload it to photobucket and link to it from there. The sounds files will be too big for the forum to host them for you, sorry.

Cheers R.

[pingu]

You may be able to upload an audio file to youtube. Never tried it.

If you can't, record it as a video on your phone and upload the video file.

[edgeshift]

It was recorded on my iPhone, it's eight mins long but I can trim the relevant sections out.

[Robert T]

I was suggesting photobucket as they do allow you to upload video and you already have an account. The help was a bit sketchy about hosting audio.

Youtube explicitly states that you have to convert files to a video format before you can upload them (see support.google.com).

Other than that, dropbox or similar would work.

Cheers R.

[edgeshift]

Not been able to get the car to a garage but noticed a few other facts about the problem.

When I accelerate the noise goes until I lift off the throttle. It is there when coasting and under braking.

I've tried taking of each wheel and the rear driving side wheel is stuck on! Any suggestions on how I might prise it off? This could be the problem but cant see anything physically wrong, could be just corrosion? and not related at all.

[mnbrennan]

Probably corrosion. Get a rubber mallet and hit alternative sides of the rim until it comes off

[Mike Fishwick]

Wrap a towel around the centre of the wheel, including the rear face. Now slowly pour a kettle or two of boiling water over it - this will expand the alloy, and the wheel should come off easily. Then clean the wheel bore and the hub spigot, lightly coat them with Copa-Slip, and repeat on all the other wheels. It is a sign of neglect - but is common, not being included in the wonderful 'Full' BMW Service schedule . . .

[edgeshift]

Thanks mnbrennan, will do, nice car.

[edgeshift]

Thanks Mike

[Brian4]

If you use the rubber mallet don't have your head/face too close as the hammer may bounce back at you off the tyre. Clean off both the inside of the wheel and the spigot on the hub and small amount of copper grease.

[XVar]

I once had the exact symptoms that you describe, a knocking noise present under coasting and braking but not under acceleration, which also sped up with the speed of the car. It was diagnosed as a failure of the dual mass flywheel - this was on a 307 TDi so a completely unrelated vehicle but the symptoms were literally identical. Do you notice any shudder when you press the clutch slightly just to the bite point?

By the way, SoundCloud is good for uploading audio for free: https://soundcloud.com/

[pingu]

Wheel just off the ground and free to rotate.

Block of wood to protect the rim.

Large sledge hammer (7 lbs) and swing it at the bottom of the wheel.

Rotate 180 degs - another hit.

Rotate 90 degs - another hit.

Rotate 180 degs - another hit.

Rotate back to start position and repeat until the wheel comes off.

Clean the centre of the wheel and the hub. Copperslip the centre of the wheel, the face of the hub and the bolts. Torque the bolts to 100Nm using the correct pattern for a 5 hole wheel.

Hint:
One of the first things you should do with a new car is take the wheels off and make sure that they are easy to remove at the side of the road. Better to find out it's stuck when you are at home than when you are at the side of the road in the rain.

[Mike Fishwick]

Remember that when hammering your wheels off you are also hammering your hub bearings, which is not a kind way to treat any bearing, as it will cause vastly amplified loadings on the contact point between the balls and races, causing premature failure.

[pingu]

Remember that when hammering your wheels off you are also hammering your hub bearings, which is not a kind way to treat any bearing, as it will cause vastly amplified loadings on the contact point between the balls and races, causing premature failure.

Point noted, but I've never had a problem - and I've had some VERY tightly attached wheels to remove.