I have a 1998 Chevy Sub. 1500 4x4 that makes a loud tapping noise after it runs for several mins. It has 150k miles and we are the original owner. After the engine is warm, and tapping, there is a noticable lack of power. There is also a sound very similar to a whistling or whine coming from the area near the passenger's side firewall. Could this be an indication that one of the catalytic converters is bad? It runs fine until the engine is up to temp.