Fast idle cycling from 1500 rpm's to 2500 rpm's on 2002 Chevrolet Blazer

Had a intake gasket failure and replaced gaskets from the heads up. Started up and idle was so high I was nervous it would over rev. Also have a knocking sound like a exhaust leak at first and then goes away after about 30 seconds of running with the cycling fast idle. Thanks!

Asked by for the 2002 Chevrolet Blazer