High pitched noise on 2004 Acura RSX

I have a 2004 Acura RSX with only 40,046 miles on it. Recently I have noticed a high pitch sound while driving. I thought it might be the automatic transmission but it changes gears smoothly. My mechanic couldn't tell where the sound was coming from and thought it could be in the engine. He suggested I wait for the sound to get louder. Has anyone else with this model RSX had this problem or do you have any thoughts about what it could be? Thanks

Asked by for the 2004 Acura RSX
I would put the car up on a lift and spin the wheels up to the speed where the noise is happening, then use a stethoscope to pin point the source of the sound. This is what I do, when I have this type of problem and it works very well.