I was driving on the highway today and noticed two things I wanted to ask about.

First, I had my a/c on and when I revved around 4000rpm I noticed a high pitched sound coming from the front of the car. When I turned off the a/c I didn't hear it. What could that be?

I also am hearing a sort of humming sound around the rear of the car at speeds over 80km/h. It almost sounds like I have heavy tread tires on...I checked my tire pressure and its not too high. Someone suggested my wheel bearing might be going...Please help.