Over 4th of July weekend I took my wife down to the Jersey shore. On the way home I started noticing a very quiet sound like a rapid hollow metallic thumping. It was minor and I didn't pay attention to it. Fast forward a few months and this sound is still there. I can feel a vibration in the clutch pedal but nothing else so I'm assuming it's the TOB. Clutch has about 5K miles on it now Spec stage 3.
One night I decide to get on the go pedal for the first time in months in a quick 2nd through 3rd pull. Immediately after I slow down and begin accelerating normally I hear something else. Now at anything over 2K rpms to 4K rpms I hear what seems to be a new noise. It's like a hollow warbling sound accompanying the thumping. Like an exhaust leak or something. Out of gear and cruise it's there as long as the accelerator is being pushed. Power feels a little down too and the turbo spool feels different, though the turbo shows no shaft play whatsoever. I'm afraid to really get on it, but it looks like I'm only pegging around 18 PSI vs the 23 I usually run. With the windows down you don't hear it. It's mostly noticible with the windows up while accelerating. Obviously could be a cracked manifold, but there appears to be no leaks from that.
I haven't tore into much yet (have a newborn at home and work a lot). Next week I'll have 4 days to myself to go over everything. Compression is great, and hacking the oil filter didn't yield any metal shavings so it's not likely a rod bearing. Nothing appears to be hitting anywhere. Motor mounts are fine. It rises and falls with the RPM's and isn't there when cold. You have to drive a few minutes to get it to happen. I suspect a really bad TOB as that's clearly rattling at idle and the pedal engagement got weird on me suddenly though that could be unrelated. The TOB didn't start making any noise until the trip to the shore. I'm not yanking the trans on a whim unless it's the general consensus though. I plan on having Shep rebuild it this fall/winter sometime and I'd hate to pull it twice. Anything and everything you can think of that I should look at, please let me know so I can have some kind of game plan.

 

It really could be the TOB (throw out bearing) or the carriage bearing on the drive shaft especially since you mentioned the "thumping" sound. I don't have the thumping sound yet but I am led to believe that a rattling I have under deceleration maybe either of the 2. I'm kinda leaning to the TOB since the sound seems to come more from the front than the middle of the car. At first I checked exhaust thinking that was the sound but could find nothing.

Good luck finding it........I know it's a PITA.

 

A massive PITA. I can't pinpoint the sound at all. I listend to the oil pan and trans case through a long screwdriver (works like a stethascope), and I could hear the distinct throw out bearing noise, but nothing else. I have a mufflerless 3" exhaust, so revving it up at idle to pinpoint it's whearabouts just drowns everything out and I can't hear anything at all but the engine. I did have the rattle for a while, and the standard thump under deceleration, but thinking about it, I haven't heard that at all since this noise started. Hmmm.

 

Check for play in the carriage bearing, it may well be your culprit. I don't know how much is too much but I don't think it's to move around too much.