Popping sound from left rear - 330i

[330isedan]

I've been experiencing a ''popping'' / ''clicking'' sound for 3.5 years now and it's driving me crazy. I went to 3 mechanics and they just can't find it. It sounds like it's coming from the left rear area of the car.

It occurs everytime when the car slowly gets in motion from a standstill, but it's also really noticeable when driving slowly in second gear. When I apply a slight amount of throttle, it pops a few times. If I drive very slowly and shift from 1st to 2nd gear, I can hear the popping sound as soon as I release the clutch and apply some throttle, but after those pops (and still while slightly accelerating) it doesn't click anymore untill I release the throttle. It looks like it doesn't occur during hard acceleration from a standstill, or when pressing the throttle all the way in second gear, but only during light acceleration...

Extra useful information:

• The car is 330i 5spd manual from late 2000.
• Subframe mounts are all fine. They have been checked for cracks multiple times.
• The subframe bushings have been replaced
• Rear trailing arm ball joints have been replaced
• The diff bushings have been replaced
• Rear Anti-rollbar and links have been replaced
• Shocks and springs have been replaced (slightly lowered by 30mm)
• The CSB and guibo have been replaced
• The spot welds in the trunk just above the 2 rear subframe mounts have been reinforced and that area has been checked for any cracks.
• Tires and rear brakes are fine.
• The sound is NOT related to bumps or corners and the handling is not affected. It looks like it's related to the torque from the engine as the sound is mostly noticeable in the high torque gears and in low RPM range.
• I do have some vibriations at high speeds (+200 kph)
• I'm not sure if it could be related to the driveshaft or diff. The u-joint on my driveshaft is not as far back as where the sound appears to be coming from. The CV joint between drivehsaft and diff is further back, but is less likely to fail from what I've read.
• It could be some play in the (left) halfshaft. I know there is some play along the axis, which seems to be normal... Last time I checked there was no rotational play. I don't know if a bad halfshaft would result in this popping sound.

Since to me it doesn't sound as if the sound is coming from a rotating part, what else could it possibly be? One of the bushing that I haven't yet replaced, is the rear trailing arm bushing (number 4 below). Could this possibly be the source of the sound?

I added 2 soundclips of the sound. I hope that will help.

https://youtu.be/kLE_xqF02m8

https://youtu.be/r7_zoGptJw0

[E39 Newbie]

The RTAB fail more often than the other parts you mentioned. You replaced the big bushing at the back of the diff, too, right?

Get under the car with a prybar and pry the RTABs. Do they move a lot? Are they soft or cracked or split?

[330isedan]

Yes the big bushing at the back of the diff has also been replaced.

When the car was on a lift my mechanic and I saw that the rubber edges that stick out of the RTABs look very dry and cracked, it basically falls apart when you rub your finger against it... He said they should be replaced, but when he used a prybar on it he said the movement was actually not that much. I don't know if I can trust my mechanic anymore, after all the things that he told me... (replaced the ball joints, diff bushings, subframe bushings, he told me it could be the RSM (it's definitely not the RSM, I checked it), and when I told him the diff bushings did not solve the issue, he told me it could be the propshaft).

If the edges of the RTABs look dry and cracked, I assume the RTABs are done. Apart from the sound, I don't have other symptoms of bad RTABs. The sound does not occur on bumps, which I would expect when the RTABs are done, but you never know. I will replace them next month and I'll keep you updated.