[QUOTE=Artemis;20291620]
Quote:
Originally Posted by gmzanatta
I already heard many times (30 times or so in 7000 kms) some strange grinding noise coming from the back underneath.
It's a screeching metallic sound of some metal part rubbing against another metallic part for more than an instant. A kind of metallic rambling (or some rambling inside a metallic box) for a few moments (a second or so) rather than just some brief clunk. Happens at random, sometimes twice right after each other. Mostly when the car is not under load (for example cruising at slow speed or after coming out of a turn). It sounds as if some part moves in a controlled way from one side to another (opening or closing of a part) but with an awful rambling noise. Always exactly the same sound. I even got used to it ("Oh, here it goes again"). As far as I experienced, no impact on the driving itself.
Very weird. Never heard that on any ///M car I ever drove. And no, it ain't the opening/closing of the butterfly valve of the M Performance Exhaust (totally different sound) nor the notorious 'cow bell sound' phenomenon caused by pebbles that got inside the brake system. I suspect the diff performing weird stuff.
I told my dealer. He reported it to BMW. He saw in the system a notice about (normal) screechy sounds at slow speed during break-in: bedding in of the brakes during the first 500km. Definitely something else.
My car is an early April 2016 build.
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I've heard it myself and also heard it on my M4. I am almost certain that what you are hearing are pebbles lodged in the dust cover on the brakes. They screech for a second and then fall out. I could be wrong, but it doesn't sound like a cow bell and I'm sure it's this. Also, happens sometimes driving in a straight line where the diff would not be relevant like it would in turning.