Got a 99 4.2 with 157,000 on it. I've taken good care of it and knock on wood never had any problems with it. The last few months I've noticed a knock or tap coming from the bottom of the engine. It's very inconsistent in that sometimes it's there and sometimes not. When it is there gears have no effect, it does it at idle and in drive and speeds up with the rpm. The engine runs good and strong. I've read a couple of things that made me think it could be the cat. converter. From what I can tell the tap is coming from that area. Does that sound possible. I've never heard a cat make that kind of noise before so I'm really not sure. Is there any way to check for sure without actually pulling the cat. Thanks

 

A muffler shop can test the cats for blockage with an infrared sensor.

 

i have been getting an inconsitent knock as well but seems to be by the front driver tire.. when this happens the truck vibrates alot but not when idling i was thinking it was the 4x4 sticking so i would mess with the shift a little and it would stop but last week i hadnt used the 4x4 andi got in in the morn and it did it again..