whinning noise in engine.

[Shoki]

Hi,I am recently hearing a whinning sound from behind of one of the pulley's in my SR20. either the water pump, power steering, and alternator pulley. when the engine is warmed up then it stops. my mechanic said at first that it could be a bearing.a day later he noticed that the oil pressure was really low when starting, warmed up, idle. about 25-35psi. he is looking into this now to see if its not just the gauge or something. this just started happening recently so i decided to not drive it untill its worked on. but anyways, does anyone know what this could be?could it be the same problem?i searched this forum for something like it, and couldn't find anything.

thanks a lot.