Not saying this is the problem you're experiencing, but it's what I had happening: Try inserting your coupler into your receiver.....no trailer...inset the pin, then grab the ball & yank up & down. Any movement? That may very well be the clanking you're hearing.
My coupler (class III) was fairly loose in the truck's receiver, and that resulted in a lot of "clunking & banging" as the coupler "pivoted" up & down around the lock pin. Not a lot of movement, really....but enough to be quite noisy while under load. Tongue weight didn't have much impact one way or the other.
I know this will sound like a BS solution, but a single wrap of duct tape around the inserted end of the coupler cured the issue 100%. It tightened the coupler in the receiver "just enough" to reduce almost all of the motion and deadened / dampened whatever motion remained, preventing steel-to-steel banging.