A/C Squeal

When I got my head gasket replaced, my car made a squealing sound when I ran the AC, but only once and I did not think anything of it. Now lately, whenever I start my car cold, if I try to run the AC my car squeals. Once it has been running for about thirty seconds it can idle with the AC on and no squeal, but as the rpm's go up, it squeals again. As the engine temp goes up, the higher the rpm's it takes to hear the squeal. Once it is at normal operating temperature, you can redline without hearing it. Any ideas if its a belt or some AC component?