What can cause squeeky brakes?

About a month ago, I change my front rotors and pads and changed my rear pads. However I still get an annoying squeel when braking every once in a while.

I cleaned everything changing my breaks, but I did not replace the rear rotors. Can old rotors result in squeeking? I'm sort of at a loss here.

Is the noise from the front or back?????? Did you regrease (white lithum) the caliber bolts and where the pads(metal part) slide in the caliper???

For the rear, scuff up the rotors with 100 grit sand paper(both sides) and see if that might help.....

Is the noise from the front or back?????? Did you regrease (white lithum) the caliber bolts and where the pads(metal part) slide in the caliper???

For the rear, scuff up the rotors with 100 grit sand paper(both sides) and see if that might help.....

I don't think I greased the caliper bolts, just the pads. I'll pop'em off again and see if that does the trick. I'm having a heck of a time telling if the sound comes from front or back, initially I was guessing the back, but the squeek doesn't happen at low speed very often, but happens more when say I'm slowing from 45 to 30 and the squeek sometimes sounds a little inconsistent if that means anything.

i put on Hawk HPS+ pads front and rear and now all 4 squeak. stops like a mofo though. i also did not re-surface the rears, but i did the fronts cuz they were an LS1 swap