intermittent motor knock

[00RangerShavedAndDropped]

i have a 00 3.0 4wd with 97xxx i just bought it. told there was a bad head gasket but still trying to figure that out my question is what would cause a random motor knock. a couple times when i started it. has happened both cold and warm i heard something like a motor knock. and same thing when turning it off sounds like a motor knock when turning off any ideas what could cause this

 

[Earl43P]

See if it does that with the heater/AC Control selected to OFF when starting/shutting off.

 

[00RangerShavedAndDropped]

it has always happened with the ac off i never have it on when i start the truck or turn it off.

 

[00RangerShavedAndDropped]

alright so it does it all the time when i rev it fast in park. and anytime i rev the motor really hard it knocks any ideas anyone anything i should try