How do I know that I need rotors replacement

[jb300sel]

My 300 SEL 1988

When I am reversing from the driving way there is this knocking sound on the rear right wheel
When taking off (forward) this knocking sound increases when the wheel turn faster. It's loud as if I am using a hammer.
When applying brakes it's like its grinding (metal to metal)
At times this sound is not there at all.

I checked the break pads and they are in good shape.

Could this be the rotors.

I tried following the discussion on the rotors but it's not that clear.