I have an e36 325i (slushbox) and when I accelerate moderately or hard I get a serious thumping/knocking between 20-30mph. It doesnt happen at any other speed regardless of how fast/hard I am driving.

I was thinking that perhaps it could be the bearing on the driveshaft (if feels like its coming from the lower right to a driver and lower left to passanger).

Find a and accellerate briskly from a standing stop. If you get a pronounced thumping from under the car at low speed that dissapears as the speed increases, you probably have a failed center bearing and bad driveshaft. The reason that the noise dissapears with speed is that the torque load on the driveshaft decreases with speed.

In almost all cases a failure of the rubber mount for the center bearing is a result of binding u-joints in the driveshaft. Once the driveshaft is out you can inspect the u-joints for even the slightest sign of binding or wear. The u-joints aren't field replaceable, so you'll need a new or rebuilt driveshaft and a new center bearing. And when replacing the driveshaft a new guibo is recommended.

A visual inspection of the driveshaft will tell if the guibo or center bearing mount is bad.

you need a new guibo, its what connects the transmission to the drive shaft, its like a rubber/urethane to absorb shock and shit, its something that needs to be replaced every now and then

I checked the guibo. Its doesnt appear to be bad, what would be an indication of failure ? (I also checked the diff fluid and the condition of the boots on the half shafts)

well mine was fucked up, but all it takes is for that thing to be weak to allow the driveshaft to go out of wack, mine was hitting the well around it.

look for cracking, shaved off parts, just general wear on it. i would get the same knocking sound in the middle of the car like you described when i had it, except it was scarily loud.

In my experience, a failed quibo causes a vibration that gets worse with increasing speed.

A thumping/banging sound from more or less the center of the car that only occurs when the drivetrain is subjected to high torque loads is most likely to be as a result of failing u-joints in the driveshaft.

The signs of a bad quibo are cracks or signs that it is shedding material.