engine noise

I'll get straight to the point..I recently noticed a noise on my 96 328 is that occurs only when my car is in gear and gets louder when i turn my headlights on and i don't want to take it to a mechanic and have him charge me a million dollars for something small.....i tried to locate the problem myself and it seems to be coming from the alternator or the pulley on it....other than this noise the car is flawless.i have recently replaced my belts and am wondering if this noise is related to that?
Does this noise sound familar?

rub a peace of bar soap on the v of the v-belt for the alternator when the engine is not running.

If the noise goes away after applying the soap, you need to tention the v-belt.

Later,