I've got a '99 Altima, 5 speed, that just turned 96,000 km.  In the fall I
had a new clutch installed by the dealer, and in January, all new bearings
put in by a well-reputed local transmission shop (this to solve a whining at
highway speeds).  Apparently the previous owner didn't know how to drive a
stick.

It's been working well, but I've noticed since the weather has been warmer
that once the car has warmed up, there's a funny sound in neutral with the
clutch pedal out at idle.  It goes away as soon as I press the pedal in.
It's not exactly a grinding, or a whirring sound, the best way to describe
is maybe as a buzzing combined with a whirring sort of sound.

As I rev the engine slightly up from idle, to say 1000-1500 rpm, the sound
disappears and/or is drowned out by the engine, it's hard to tell which.
What's really interesting, is if I press the clutch in, then let it spring
back quickly, the sound goes away, until I press and release the pedal again
normally.  Sometimes I'll get the sound intermittently after releasing the
pedal, and it'll only happen 2 or 3 times each second, increasing in
frequency as I rev the engine.

I had it back to the shop that replaced the bearings, and their opinion was
that the noise is likely caused by the gears having worn slightly while the
bearings were bad, and the sound is of the gears meshing.  This sounds
possible, but what gets me is the fact that I can practically eliminate the
sound by popping the clutch in neutral -- does that make any sense??  I've
checked the fluid level and it's reasonably high, but it could take a little
more.
The shop claims that they would have used 80w90 fluid, which is the thickest
they carry and what the manual calls for.

Any suggestions are welcomed, thanks in advance!