My '01 Supercrew w/5.4L auto and 28,000 miles has a strange pinging and I wonder if anyone else has encountered it. I hear it most when "lugging" my engine, as with any pre-ignition type of ping, but also at highway speeds. My dealer, of course, cannot hear anything. They also found no codes and the anti-knock sensor checked out OK. The problem began at around 22,000 miles and has not stopped even when I installed a Gibson Cat-Back around 2000 miles ago. Any ideas?