R30

My husband and I replaced the belt, but didnt know what to do next....

I have a 1994 Lexus GS300, when I turn the key it makes this awefull noise but nobody seems to know what is going on...at first it sounded like it was the timing belt, took it off and then replaced it...same noise still existed..its like a screech..it drives fine, the noise dosnt exist while driving except when I turn the air conditioner on ..then its constant ...(although at 199,000.) Maybe it could be the air compressor?? Im no mechanic..and Id hate to spend a lot of money to a garage if "they cant seem to find the problem" or "well have to take it apart to see whats wrong".... Thanks for all the help...