Sometimes when i get into reverse and start to clutch-out, the transmission makes this highish pitched noise, like either a whir or a high-frequency light grinding. It goes away after clutch in, and only occasionally occurs, usually at night when the car has been sitting for at least 5 minutes.

Does anyone else get this? What is it?

Synchro noise? (Not sure if the '8 has synchro on reverse but I know some Mazda's do).