I have this same exact problem. If I'm going under ~20mph and am under 3k rpms it will occasionally make this noise.. It kinda sounds like a humming noise, but it's hard to explain. As soon as I push in the clutch, or if the car is out of gear, it stops. Also when it starts making the noise, if I give it just a little bit more gas, it stops. I've asked before and no one could tell me what it could possibly be.