Groaning?? Brakes

Hey all have a problem here. I had a shop do the front and rear brakes for me about a month and a half ago. They Resurfaced the rotors, and put new pads on, up until last week they were great, but now, after coming to a complete stop, or even when stopping as I apply pressure I can hear a groaning sound coming from the fronts, imagine a squeeky shock on an old vehicle or something and that is what it sounds like. No it is not a screeching sound like the pads, or anything. My guess is that the pistons in the calipers may need to be greased up and this will solve the problem. The brakes work perfect, smooth, and do not heat up badly at all. They were broken in properly etc. Like I said its just a groaning sound as I am applying pressure to the brake pedal. Any Ideas? Does my theory sound correct? I am going to take it in to Cadillac this week as I am still under warranty but they shouldnt make a big deal that I had the brakes done at another shop right?