You have reminded me I did once experience a slightly similar noise on a previous car, which after trying umpteen things finally went away by itself, so I never knew what had caused it.
(Read the saga here -
https://www.volvoforums.org.uk/showthread.php?t=229118
but mixed up with another unrelated noise)
Another thread did highlight a soft gearbox rubber mounting, causing the box to jump about under starting torque. It was hard to spot until it finally collapsed and the box was sitting on the cross member.
I've just remembered another similar noise, caused by the gear lever carrier banging on the side of the bodywork turret.
Despite new engine and gearbox mountings I could see when lying directly underneath that the back of the box was not central in the space, and could move enough to touch the metal on one side.
In the end I removed the mounting bracket and elongated the fixing hole to a slot so that I could centralise it properly.