I have a pair of 1 year or less old slotted rotors and pads with a good amount of thickness on the fronts, where the noise is coming from, and have never had this problem until recently. Only thing I can think of that is different or could have altered anything is that I had the brake fluid reservoir cap off for maybe 20 minutes, and then covered with a piece of tape for about an hour while I painted it black (no paint got inside).

What could be causing this?