I am having a problem of what I think is a valve tap. Whenver I first start moving the engine will not tap. If I sit in traffic for awhile the engine will tap quietly. If I drive a long distance on the interstate around 100 miles or so, whenever I am done driving my car sounds like a diesel when sitting still because tapping is so loud, also whenever I start moving the tap can be heard coming from the exhaust pipe also. It just sounds really bad and embarassing. If I turn my car off for an hour or so the tap will go away. Can somebody help me figure out what this is! I drive the 93 I4 manual.