98 Ranger 4.0L engine knock

Bought this V6 used. Dealer said problem was deposits in engine and they kept it to "burn out" the carbon deposits. Problem came back after a few hundred miles. Lo mileage - 70,000. Knock sounds like a diesel at times. Does not get worse under a load and is evident at idle.

Any ideas?

Check plugs,wires,dist. cap and rotor. Also could be incorrect timing. Just some ideas.

All that was checked by the dealer before we got to the burnout procedure.