With heater on highest setting, the engine sometimes knocks loudly. Had radiator flushed last year, blows hot, then very hot and engine starts knocking. Any suggestions?

Check your thermostat. It may be sticking closed.

You flushed the radiator on the inside, give the outside fins a really good deep cleaning, check to make sure your cooling fan in coming up to speed...

Would a stuck thermostat cause the engine to "Knock" loudly?

Finally located problem....bad a/c compressor clutch. When engine started knocking/clanking, I pressed the a/c "On" button and noise stopped. Turning "Off" a/c , noise would begin again! Verified by local dealership. Now the repair job will total roughly $870 before taxes!!