Ok, so I installed my slotted rotors and axxis pads about 4 months ago. They have been great until about a month ago. I noticed one day after it rained, and the rotors rusted, i went for a drive, and when i got back i noticed that the rust hadn't got cleaned off the rear drivers side rotor. now, when i did my brakes, i had to get a new caliper on this corner because the stock one was seized. i put brand new pads and rotors all the way around so i figured i'd be set. it seemed like the rotor wasnt working properly.... also, shortly after this, when im in stop and go traffic, i get a loud "hum" coming from that same caliper. 

so, i took the brake apart, re-greased the sliders, re-installed everything, bled that corner....came out solid 3 times in a row....and then went for a drive. it seemed to be fixed. and now a few days later its doing it again.

anyone have any ideas how to fix this? one thing was to maybe sand the brake pad down a lil bit...but idk. its wierd idk what the hell the problem is. everything seems to be working fine, but im getting that humming noise real bad. its really annoying and i need to fix it.

please help!!!