Small Engines (Lawn Mowers, etc.): LOW IDLE CLUNKING, ohv motor, head gasket

Question
I HAVE A MTD WITH A B&S 15.5 HP OHV MOTOR MODEL#28N777
TYPE#0642A1 CODE#961010ZD JUST CHANGED HEAD GASKET DUE TO
SMOKING,LOW POWER. I HAVE A CLUNKING NOISE ONLY @ LOW IDLE
COMING FROM MOTOR ALSO CLUNKS WHEN SHUTTING DOWN. WHAT COULD BE CAUSING THE NOISE? I HAVNT TAKEN FLYWHEEL OFF, I
DID REMOVE COVER AND BOLT TO FLYWHEEL KEY SEEMS OK FROM TOP VIEW. THANK U IN ADVANCE  JOE

Answer
You are probably hearing the compression relief lever on the cam.  Speed the idle up above 1100 rpms and this should go away.   This engine hates heat.  The quickest way to overheat the engine is to let it idle.  This engine needs to run WFO (Wide Freakin Open) every second it is running.  From start to shutoff.