I have a 2006 350z, it was rolled in a wreck.
We got it to run with no thuds or unusual sounds. We switched engine and transmission into a new body along with ECM, BCM and key cylinder. Now there is a thudding coming from where the transmission and engine meet. The noise disappears when the clutch is in engaged or put in gear. It shifts and drives fine. Ive given audio for the test drive and idle thuds.

What could this be?


Edit: I found out it was low in oil and the previous owner upgraded the clutch and flywheel. The flywheel is lighter so it louder than stock.