411 Engine noise

[Hunter]

Hello Forum

We have a 411 with the Dodge (Chrysler) 383 engine.
We discovered knocking coming from the drivers side rocker cover. Once we removed it we found the centre rocker arm had been hitting the cover, leaving marks in the aluminum covers. The cover doesn't seem bent or misshaped other than the marks.
The rocker arms all seemed in place, i couldn't push the lifters down so all seemed fine. Ran the engine without covers. no noise
I replaced the gasket with 2 rubber gaskets, refit.
I run the engine again, and again the knocking.
The knocking continues even when hot and seems to stop at about 1200 rpm.

Does this seem like a problem anyone else has had?
Any help, or ideas welcomed.