Corolla / Matrix 2009-13 :: Intermittent Front Brake Squealing

Basically, whenever I brake, my front brakes squeal. But they only do this if I use about 25% to 33% force. If I press on the pedal harder, the squealing goes away and the same if I reduce the force.

I replaced my pads with Wagner ThermoQuiet pads in March 2015. The noise started a month or 2 after replacing my pads, and I've only put about 12k miles on them. I had my car in a couple weeks ago for an oil change, and the inspection showed that the pads still had plenty of life on them. I brought it up to the service advisor, and he suggested that my rotors might be warped and need to be machined. The problem with that is I feel absolutely no vibration in my brake pedal that would indicate warpage.

So the only other thing I can think of is the shims might be causing the noise. I was going to get some grease and apply that and see if it works. Also, when I was replacing the pads I bled out a little more brake fluid than I intended. But it hasn't seemed to affect my braking ability so I ruled that out.

Any guesses for what might cause this? Like I said, I have plenty of material left on the pads and I can stop without problems, so it's not a safety issue, just a tad annoying.