2006 Chevrolet Silverado • 75,000 miles

I have a 2006 chevy silverado 1500 4.3 v6. I had two check engine codes. The first one was for the oxygen sensor in bank 2. I replaced the sensor and the code went away. The second code was from multiple misfires. I changed the cap, rotor, and wires and this made that code go away. Now there is a loud pinging noise coming from the passenger side of the truck. It seems to be coming from the general area of the flex plate or y-pipe of the exhaust. The noise is not present at idle. Only during the deceleration of a rev does it start. After driving for a bit it will go away. There are no check engine lights on anymore. What could cause the pinging noise?

June 1, 2012.

Have the torque converter bolts checked for being loose. Have them also check the flexplate for any cracks in the center.