Originally Posted by Savannah Buzz

Another shade tree method - with engine off, attach shop vac output to exhaust pipe, plug other (if duals) with potato (yep, that still works you little pranksters) or duct tape. If you can't hear the leak, a cloud of cigar smoke blown in suspect directions may suddenly be swirled by the escaping air. This way you are not in competition with the truck noise and radiator fan. Sometimes pipe leaks can be located by shoving a boat canned air horn in the exhaust wrapped with a cloth towel to keep noise in the system.