Help, strange noise !

[Wolf_Tm250]

Hello,

I'm rebuilding my engine now, out of the car.
When I turn crankshaft I hear 2 "clunk" noises, one is at #1 and #4 TDC and the other is some degrees later.
Given that the engine was not making this noise without the gearbox, I loosened the gearbox main bolts ( only loosened without moving away it ) and it disappeared !
The FW/clutch is an HKS twin disc, but I looked at it and it seems not to touch anywhere the box.

Please help me, bye.