E46 Knock under car?

[V1L3]

Hi all,

Looking for some advice again.

I have an e46 i'm in the process of turning into a track car, we now noticed that when you put the car in gear, and accelerate (lightly or hard) there seems to be a knock or a "doef doef" sound coming from under the car., from the back of the car to be precise.

I had the car on jack stands and tested this, it seems there is this sound when putting the car into gears but then once you release the pedal the sound gets worse for a moment.

Just some other info that might be needed.
The car has a new propshaft that has been balanced with new donuts etc and everything is tight including the diff.
the clutch has been bled and everything works fine on that.
The car was a non runner and i'm not sure how long it was standing for before i got it if that might mean anything

If anyone has any ideas what it could be i would really appreciate it.

 

[Nikhil]

Rear diff mounts more than likely

 

[Nikhil]

Infact the side shafts also sound like a plausible cause aswell

 

[Iceman007]

https://www.turnermotorsport.com/t-gallery-e46-subframe.aspx

Maybe Subframe failure

 

[Crash_Nemesis]

Subframe failure / Boot floor coming loose due to pop rivets breaking. Take out your carpet/matt in the boot and your rear seats. Look for cracks in the corners between the boot floor and the rear seat.

I had the exact same issue. Fixed it but it isn't easy.

 

[Philip Foglar]

Since you are in the Cape Town area, maybe worth taking the car to Precision Wheel Alignment in Parow - they know the E46 very well and I often see track cars there, so I am sure they can help! Speak to Ian there! Sadly this does sound very much like the above, cracked subframe etc.

 

[zippy320]

As the above posters have mentioned , Could be the read subframe , Could just be that the mounts are gone , or could be cracks .

 

[V1L3]

Hi all,

Thanks for the comments,
I have checked the subframe and everything looks good there, also we had the subframe issue on an e36 race car before and its definitely not that with this car.

@Nikhil - I checked the diff mounts and the side shafts as well and everything seems fine there as well.

 

[Crash_Nemesis]

Did you check the boot floor? The pop
Rivets come loose sometimes.

 

[Blert]

Prop hanger?

I had mine replaced what happened was under load I would hear and feel the car knocking at the back. Once moving the sound goes away

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[Pho3niX90]

I also say diff mounting. Is it only on take off or any type of acceleration? If the latter could be the propshaft center bearing

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[V1L3]

I checked the prop hanger and its seems all good, also it seems to be under acceleration and then when i lift off it shudders again.

Checked the diff mountings they seem a bit perished but not broken or anything so gonna try find some new or newer ones and see if that helps

 

[Crash_Nemesis]

Check your rear shock tower mounts. Is this an e46 m3? If so, I had the same issue man. As you accelerate, there is a knock, as you stop accelerating, there is a knock. My issue was the boot floor pop rivets coming loose. I still suggest you check your boot floor.

 

[Pho3niX90]

I checked the prop hanger and its seems all good, also it seems to be under acceleration and then when i lift off it shudders again.

Checked the diff mountings they seem a bit perished but not broken or anything so gonna try find some new or newer ones and see if that helps

Did you check if the diff moves?
I remember from visual inspection all looked fine, but when trying to move it with little force you could see the mounting was hectically damaged.

Also the center bearing:

While you are under there, have a look at the guibo as well

 

[V1L3]

I checked the prop hanger and its seems all good, also it seems to be under acceleration and then when i lift off it shudders again.

Checked the diff mountings they seem a bit perished but not broken or anything so gonna try find some new or newer ones and see if that helps

Did you check if the diff moves?
I remember from visual inspection all looked fine, but when trying to move it with little force you could see the mounting was hectically damaged.

Also the center bearing:

While you are under there, have a look at the guido as well

Thank you so much for the pics,
So i checked again, the dif is moving under acceleration, i would imagine that would be the thumping noise?

The hanger and Guibo are all new so it cant be those, but the diff definitely has play in it.

 

[Nikhil]

Correct so as stated initially diff mounts. :coolShake:

 

[Pho3niX90]

Yeah the play should be minimal. I would replace it soon. And while you are there drain the diff oil and replace with new oil

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[V1L3]

Guys, thank you so much for everyones replies.

I will be changing mounts next week and will revert back with feedback, again, thank you all!

:ty: