Noise from amp question

[Rolling Estonian]

The last couple months I've been hearing this soft high pitched whir sound that goes up in pitch and loudness as the revs of the engine go up, such that at idle it's barely audible and at 4-5K rpm it's quite noticeable. At first I thought it was something from my transmission, and had plans to have a mechanic look into it the next time I was at the dealership.

However I turned my head unit off while driving today and the whir stopped, radio back on whir is back. I'm now thinking it's related to my charging system and the current my alternator is putting out (higher engine revs faster the alternator spins, hence different sound at different rpm)

I have a 500W single channel and 400W 4 channel Rockford amps. I have a 4 gauge power wire, no big three upgrade yet.

Any one have a possible explanation.

[EW]

Unplug the RCAs on the 4 channel and then start the car.

If the noise is still there, it is the amp, induced into the crossovers or speaker wires, bad ground, etc. Also, you can try grounding the RCA shields and if the amp is screwed down to the chassis of the car, you can remove the screws and isolate the chassis of the amp from the chassis of the car.

If the noise is gone, the noise is coming from in front of the amp like RCAs, deck, etc.