I've noticed over the last couple of years that I've had my truck that whenever I run the A/C, it sounds like my truck has a horrific case of spark knock coming from the engine compartment. Is that what it is? I have noticed that its not nearly as bad when the A/C is off if it's even making any noise to begin with. I don't know anything about the A/C system, so I don't know what might cause this...

 

It could be that the A/C puts just enough extra strain on that it makes it ping. I hear it sometimes when I'm running the A/C and doing around 55-60 mph. I've been told that some knock is OK as long as it's not excessive (so what IS excessive???). I'm no mechanic, but 1. Run Premium grade gas - all the time. It's worth the extra $. And 2. Check your timing. Probably some others on this forum have even better ideas.