I just changed all four Brake Pads in my 2000 impala a week ago, but at times when I brake, I hear a high pitched noise as you would normally hear when you are running out of brake pads. But I just recently changed it, what could it be?

You may have a cat stuck in your fan belt….
Just kidding.
It may just be the new pads breaking in or you may have some dust build-up. Try cleaning the rotors and pads with brake cleaner.

Brake pad replacement