problems shifting gears

in my 2001 chevy impala, everytime i press on the brake, and my speed is above 25 mph, i hear a sound that resembles a flat tire, but i know that my tires aren't flat and it's driving me bananas. When the speed is below 25 mph, i do not hear that sound it is only if i am breaking and my speed is above 25 mph. anyone have any suggestions?

What does that have to do with shifting gears?

What exactly does a flat tire "sound" like?

Details, Details, Details...

I don't see how this has to do with shifting gears but it sounds like warped rotors. Can't tell when going slower but when going faster and hitting the brakes it shakes the whole car like crazy. Mine did that when I bought it. Had to have the rotors turned. Hope that helps.

Adam