Underbody Clunking/Knocking Noise

[Mahmood07]

Hello guys, Recently at slow speeds I started hearing clunking noises from underneath my car. Whenever the clutch disengages or when I suddenly slow down, I hear a sound that I can only describe as two metal rods knocking each other. The sound comes from the center of the car (propshaft?).

I took it in to the dealer today and he put the car up on a lift. He then turned the rear wheels back and forth with his hand and the exact sound can be heard in the center of the underbody underneath the driver/passenger seat. The SA told me the sound is normal (as usual), but it seems the rear wheels have more play than usual.

Can anybody help me out with this? Where should I tell the dealership to focus their attention on? Wheel bearings, propshaft, diff bolt maybe?

Thanks!

[happos2]

Diff bolt? Though that would be in the center of the car more under your rear seats or a little further back...

[Mahmood07]

Originally Posted by happos2

Diff bolt? Though that would be in the center of the car more under your rear seats or a little further back...

It could be, when I was underneath the car the sound seemed to be coming from the exact center of the car. Is it usual to have a bad diff bolt with 50k miles?

[jamze132]

Maybe a pebble behind one of the heatshields on the hub?

[Mahmood07]

Originally Posted by jamze132

Maybe a pebble behind one of the heatshields on the hub?

No, I'm pretty sure it's not that problem. Come to think of it the noise was coming from the propshaft if I am correct. Everytime I turned the wheel slightly, the propshaft would clunk.

[happos2]

Do you work for a aero-engine company? Propshaft...as is propellor? Driveshaft here! That gave me a good laugh as I am an aero guy...

As for comanality, not sure. But I have seen it on the boards here a few times.

[Mahmood07]

Originally Posted by happos2

Do you work for a aero-engine company? Propshaft...as is propellor? Driveshaft here! That gave me a good laugh as I am an aero guy...

As for comanality, not sure. But I have seen it on the boards here a few times.

Haha no I am not. I've always called it a Driveshaft, but recently I've heard service guys in my country calling it a propshaft which didn't make sense for me. Maybe its a US/UK thing. Thanks anyways, I will look into it!

[happos2]

Originally Posted by Mahmood07

Haha no I am not. I've always called it a Driveshaft, but recently I've heard service guys in my country calling it a propshaft which didn't make sense for me. Maybe its a US/UK thing. Thanks anyways, I will look into it!

Gotcha...no worries! Maybe prop = propulsion?! Eh, whatever!

Hopefully somebody else will chime in that knows more than me! Wheel bearings are more of a squealing noise from my experience. Good luck!

[Justinhab]

Did you find the issue?

My M started doing this today on the way to work. This happened about 6 months ago but then stopped immediately. The noise only occurs at slow speeds when I first take off. I'll have to jack up the car when I get home tonight. Hopefully it's just a rock like jamze132 mentioned.

[Mahmood07]

Nope still didn't find the issue. So far I'm just trying to live with it but it's getting frustrating and embarrassing. I'll try taking it to the dealer again and post here if I get an answer.

[OM VT3]

Yeah mine does the same I've got no idea what it is but it does come from around the drive shaft

[happos2]

Could it just be the Diff making the noise? The engine might drown it out as you accelerate. When is the last time the diff fluid was changed and with what kind of fluid?

[OM VT3]

Originally Posted by happos2

Could it just be the Diff making the noise? The engine might drown it out as you accelerate. When is the last time the diff fluid was changed and with what kind of fluid?

I've had the car on the hoist and it's not the diff

[VtecforV8]

Just got my car back from the dealer last night. They performed scheduled diff service (at third scheduled oil change after 1200 mi service) under warranty, so I would think that they would have noticed any obvious problems with the diff.

When I got back to driving it, both my wife and I noticed a metallic clunking noise (sounded like either metal popping or banging against other metal) when accelerating from a stop or coming to a stop (up to about 10mph--it was probably drowned out after that). Interested to hear what people think--the fact that a reliable mechanic at the dealer worked on the diff earlier that day leads me to believe it might be something else.

[ec_E92]

Was the "diff service" just fluid drain? Check to make sure there is fluid in the diff now. Also that noise could be loose axle bolts on the diff output flanges or the driveshaft bolts on the diff input shaft (covered by exhaust heat shield). Both are inverted torx (E16 and E14 I believe).

[Malek@MRF]

Check the center support bearing and housing. It's located halfway in the middle of the driveshaft.

And yes, a propeller shaft is the same as a driveshaft. The British call axle shafts, or half shafts propeller shafts as well as calling a driveshaft a propeller shaft.

[0_to_60]

i experienced this same issue 3 days ago. On low speeds clunk or the sound u described was present. I thought something was broken or busted. It sat overnight, sound went away

[Mahmood07]

Originally Posted by Malek@MRF

Check the center support bearing and housing. It's located halfway in the middle of the driveshaft.

And yes, a propeller shaft is the same as a driveshaft. The British call axle shafts, or half shafts propeller shafts as well as calling a driveshaft a propeller shaft.

Thanks for the tip, I'll have them check that out!

[sylonien]

I have the same sound? When I set off or slow down. Sounds like flicking a glass bottle with your finger nail or something. I'm planning to take the M to a indy to get it checked out by a pro and change the fluid and see if it makes a difference or not. And see what they say... :S

[VtecforV8]

Any updates from anyone here? I'm taking my car in tomorrow to have it looked at. I'm going to ask that they check all of the following:

-Loose or Broken Differential Bolt (maybe axle bolts on diff output flanges or the drive shaft bolts on the diff input shaft, under the heat shield)
- Check diff bushings for excessive wear/cracks; replace bushings if damaged
- Check that exhaust mounting brackets aren't broken, since they can be damaged/broken depending on how the diff bolt breaks
- If other parts are broken, confirm no damage to the rear subframe.
- Failing Driveshaft (also, check the center support bearing and housing, halfway up the driveshaft)
- Failing Differential
- Unlikely the problem, but something to try (SI B33 04 08)

[M3PO]

If it's coming from the middle of the car, it either has to be the center support bearing, center u-joint, or the exhaust contacting the center mounting point (unlikely unless you have an aftermarket exhaust).

[VtecforV8]

Thanks. I'm unmodded, save for some trim pieces. It's hard to pinpoint where the sound is coming from--could definitely be as far back as the rear axle. I tend to suspect something related to the diff since the dealer replaced the diff fluid last month (though I confirmed that they used the FM boosted fluid). They also installed a new clutch free of charge due to "a loose pressure plate", so it's also possible they didn't torque something down when they put all the pieces back together.