Grind with clutch out but not in

Hey all, 

The car in question is a well worn E36. I’m getting a metallic grinding sound coming from under the car that comes and goes on throttle. As soon as I push in clutch it stops. Shifts and drives normally otherwise.The clutch, pressure plate, TOB and pilot bearing were replaced probably 30k ago. I loaned the car to my BIL for a month or so and it came back this way. I’m planning on selling the car soon and I just need to know if this is a safety issue or something that needs to be addressed before sale. I’m not interested in dumping money into it if I can avoid it. It’s not worth it to me at this point. 

So what could it be? 

Input shaft bearing?

Transmission fluid level?

TOB

I was thinking about both of those bearings but I mentally ruled out The TOB. Wouldn’t that make noise with the clutch in? It’s silent with the pedal down. 

The pilot bearing was my other thought. I don’t know the loads on that or what would cause it to make noise/not. 

If the noise changes as you put weight on the clutch pedal at the top of it's throw, it could be the release bearing.  If it's noisy with foot off clutch, and the noise fades down as the pedal is depressed and the transmission stops turning, it's inside the trans.  Engine idling, listen and let the clutch up and down a bit.  You should hear the noise come and go with the rotation of the trans 

I spent a little more time listening and experimenting. The noise is nonexistent when the car is stationary, loudest around 35mph and seems to be worse in turns but it’s hard to tell. I realized pushing in the clutch doesn’t actually make the noise go away, just most of it, there’s still something going on with the clutch in. It sounds like it’s coming from the transmission tunnel but I can’t quite tell how far back. If I had to guess I would say between the drivers seat and the diff.

I’m leaning now towards center support bearing or maybe the differential. It’s a 3.15 lsd which for some reason I remember they had a tendency to grenade. Something about loose bolts, not sure. 

SELL,

     If you loaned it to a Family Member you don't care about it anymore anyway.

Torn trans mount? likely from a launch while out on loan. 

spandak said:

I spent a little more time listening and experimenting. The noise is nonexistent when the car is stationary, loudest around 35mph and seems to be worse in turns but it’s hard to tell. I realized pushing in the clutch doesn’t actually make the noise go away, just most of it, there’s still something going on with the clutch in. It sounds like it’s coming from the transmission tunnel but I can’t quite tell how far back. If I had to guess I would say between the drivers seat and the diff.

I’m leaning now towards center support bearing or maybe the differential. It’s a 3.15 lsd which for some reason I remember they had a tendency to grenade. Something about loose bolts, not sure. 

 

Center bearing is a good possibility.  Shouldn't really make a whole bunch of difference to the noise whether its under load or coast.  I'm not sure what bolts loosen in a differential, but if it's something that causes the gears to go out of mesh, you should be able to tell a difference in noise whether the gearset is on drive or coast.  Pinion and side bearings, not so much.  Wheel bearings, you can generally tell because they will change pitch based on which direction you are turning. Turn left, load right bearing it gets noisier.

The pitch of the noise will tell an educated ear whether it is driveshaft speed or wheel speed. Driveshaft speed noises tend to almost disappear at highway speeds.

The diffs on that chassis have two ear bolts on the diff cover, with rubber bushings, but there's also a bolt on the nose/front end.  The nose bolt is famous for breaking, and then the back half of the 2-piece driveshaft gets involved in holding the unit up.  Worth a quick look.  Jackstand job.