High idle when at normal running temp.

When my Mustang gets to its normal running temp. it idles high and wont kick down. When im at a stoplight or something it stays around 2200 RPMS and when I try to kick it down it goes even higher.When the engine first starts and cold it idles fine. I also noticed a clicking like noise coming from underneath in the front almost like something kicking on and off. I know its not the compressor. Any chance you have an idea on what it could be. THANKS!