Knocking/rubbing sound coming from transmission tunnel (e36)

[norman156]

When I bought my 328 it had a distant knocking sound which according to the seller was the front bushes. The price reflected that they needed changing so I thought that was fine. However all the bushes look fine and the sound has gotten louder, and appears to be coming from the direction of the transmission tunnel. I thought this might be the prop shaft coupling but I checked that a few days ago and it seems fine.
I've uploaded a recording of the sound here if anyone has time to have a quick listen and see if they can work out what it is. Was recorded on my phone so the quality isn't brilliant.
Any suggestions would be much appreciated
Matt

[HellDiver]

Heat shield, centre bearing, or top mount on the diff. Three common BM knocking areas.

[norman156]

Isn't the centre bearing the same thing as the prop shaft coupling? I'll check the heat sheild and see if I can check the top mount for the diff as well.

[TheEnd]

the prop coupling is a big rubber disc that bolts on the end of the gearbox, and the prop starts there. When they wear, you get transmission shunt, a lag between changing from power on to power off, and a bit of a thud.

the prop centre bearing is roughly in the middle, and supports the weight, and this will give vibrations when worn, as the prop runs out of true a little.

One of the worst knocks i had was when a prop coupling bolt worked loose, took a chunk of the gearbox case, an left the car at mach 3 on the motorway.
If you do touch the prop coupling, get new bolts and nuts from BMW. Te prop bolt's self locking feature is made by them being crushed slightly oval, and they like to strip or cross thread if reused.

The usual rubbing is from the transmission tunnel heat reflective panel corroding and falling down onto the prop

[norman156]

As mentioned I have already checked the coupling, it appears to to be completely solid, no perishing at all. I'm pretty sure I checked all the bolts but I might double check that