1999 ram 4x4,360 magnum.When the truck idles,(runs perfectly)there is an occasional sound like hitting the block with a ball peen hammer!!!!Don't know what it could be, it is just one whack,no change in idle at all.Wait a minute and it does it again. Any ideas as to the cause?It is not coming from a valve cover, sounds like it's inside the block....
It is not the ac clutch either, as it is not working at this time.

 

Is it worse if you put the engine under load?

 

no, I dont even hear it when I am driving.It is always just a single "whack" and no other sounds,no idle change,just a sound every 15 or 30 seconds when idling.
It sounds like it is coming from inside the engine toward the rear

 

Wonder if the starter sticks, or some such.... if you rev the engine, does it change the frequency?

Might be time to drop the pan, and see if anything is out of place.