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Mike, BMW Technician
Category: BMW
Satisfied Customers: 39198
Experience:  Tech Expert
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Hi I have an m3 which is 4 years old, on acceleration there

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Hi I have an m3 which is 4 years old, on acceleration there is a rattling noise coming from underneath the car - a similar sound to some small stones rattling in a tin can - what could this be?
Hello and Welcome...

My name is ***** ***** Im happy to help / advise...

With this situation, you could have small stones trapped under the car - or, loose exhaust shields - these things do need to be checked... Even at that age , you can exhaust shields / heat shields coming loose and causing this... and of course any car can get such trapped stones / debris under there too - so , a physical inspection needs to be done for these things...

Too - when under there - a physical check of the exhaust / catalytic converter and silencers needs to be done... they can fail internally so that the plates / baffles come loose and rattle inside - if the exhaust is literally thumped hard along its length - if it rattles as you describe, you have found the fault - and that section would need to be replaced.

If nothing is found, then your symptoms do as well sound to be a condition called "pinking" that can occur when the knock sensor ( or other )is failing.... for this to be found / diagnosed properly and fixed, it needs to be checked with plug in diagnostic equipment. Basically when the knock sensor / circuit is faulty, ( or other engine management fault ) you get what is called "pinking" - which is pre detonation in the cylinders - which then produces "rattling" - like as you describe - stones rattling in a tin.

Im very pleased to have helped you - Do let me know if you need anything more --

*** Meanwhile - - >>>> Please do click on the "OK/ Excellent ratings" for me - Thank you in advance for that...

You can and are most welcome to get back to me anytime you might need - clicking the OK ratings does not close you out at all..

Kind Rgds, MIKE.
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