1998 Honda Civic noises

I bought this Civic 6 months ago. The seller mentioned the new brakes he just had installed. After 2 months noise began to come from the front brakes; not a grinding or squeeking, but an intermittant sound only when brakes were applied. I took car to local Firestone garage. They said that maybe it was a couple of things, but they only knew to replace pads and turn rotors. That worked like a charm for two months. I returned and they replaced pads a second time, also commenting that they had no idea where the noise was coming from. Now, two months later, its back, and keeps getting louder daily. I had another mechanic check everything (steering, brakes, suspention) he found no problems anywhere. Any ideas? Full size new pads takes care of it, but after they get slightly worn, the sound follows. It seems to have to involve the caliper? It may also be coming from only the left front, but I can't tell for sure. Any suggestions?