strange knock..

I finished my ax-15 swap a while ago, and ever since i have a knock coming from the floor about where the bell housing should be, but it only knocks maybe 2 or 3 times and its just when i am starting from a stop when i put a load on it while letting the clutch out. The only other time i have heard it is when i hit a really BIG pot hole in the road. Otherwise it shifts and drives fine. I tightend up the tranny mount bolts and checked the bell housing bolts and they are all tight. Motor mounts are good too. I also took it to a shop for a second and they said it was prolly the tranny mount at the top of the bellhousing... until i filled him in that its just the bolts around the bell housing (including the ones that requires the special 8 star tool), and like i said, they are all tight. You can hear it, and feel in in the floor, but not through the shifter. Exhaust is tight as can be, I cant move it an inch. any thoughts???

Try putting your hand on the tc lever, it could be coming from the linkage.

Another possibility is your upper shock mount bushings.