Originally Posted by
morganc
Probably your transmission mount. You can A) replace it, or B) buy some windshield urethane and fill in all the voids around it to minimize movement.
I would recommend doing something though, because this led to my 3rd gear synchro getting destroyed a few years back. Movement in the transmission pushes back on the transmission shift cables and applies unnecessary force on the gears (akin to resting your hand on the shifter while driving).
Though, if you're claiming the noise is coming from the right (passenger) side of the vehicle, it could be A/C belt squeak. Could also be a shot passenger's side mount (the hydraulic one) causing excess movement.
The other thing that many folks here and I have installed to help with excess movement is the Corksport rear motor mount insert