Wierd Transmission Sound? HELP

[240sxss]

So lately i've just been casually driving to work and i've had no problems with the car in the last year or so... but the transmission is starting to act weird

It makes kind of like a grinding sound randomly when i have the clutch pedal pressed in all the way and i'm in any gear, most of time this happens when i slow down to a stop, i put it in neutral and stick it in first before i come to dead stop so i'm ready to take off again.

It doesnt happen all the time, just has its moments then stops, then later in the day, starts again.

I was messing around and i got it to make the sound when i had the clutch all the way in, stuck it in a gear and reved while still holding the clutch in.

Anyone know what it is?? THink i need to check my transmission fluid, maybe theres not enough?
Modified by 240sxss at 3:41 PM 1/9/2008

[S14pilot]

is it kind of a howling noice? if so, it might be your throw out bering. mine is doing the same thing. it's just a bering that engages your clutch connected to the peddle. not sure about nissans, but i know on 1st gen camaro i have you have to take the transmission out to change it.