weird brake sound/sensation

Recently I've noticed this odd sound/feeling when braking. Actually, I'm not even sure if it is a sound; it's more of a sensation that my foot can feel. It comes on intermitently. Here's a brief history:

I had a complete brake job (all pads and rotors) performed about 3 or 4 months ago. There has been no sign of brake problems (e.g., grinding, squealing, rough scratches on rotors, leaks, etc.) since this service was done. My brakes work fine, and the pedal feels firm when I step on it. My concern is this barely noticeable "pulsating" feeling I sense sometimes when I come to a stop. I don't think it's the ABS working because the pulsating is very, very weak--but I can still sense it. I can't really describe the sound (if it is a sound) or feeling. It almost seems like it may be coming from the engine or drive train, as if one of them is "braking" too.

My second guess is that it's a tire problem. My front tires are worn almost treadless on the edges, and I'm not sure if they may cause the weird braking feeling. Perhaps they are trying to grip the road when I brake. However, I don't feel any sliding; my car stops correctly and there is no steering wheel shake.

Should I be concerned about my brakes? I've checked my rotors, and they don't look scratched unevenly. My brake fluid level is fine; however, it has decreased only slightly since my brake service. I'm just concerned about this braking sensation I've noticed recently. I've never noticed it before. What's also worrying is that it is inconsistent; it happens randomly. Any comments is greatly appreciated. I need to rid myself of worries.