Hmmmm,
Lifter stuck?
Bent pushrod?
Bad rocker?
what rocker arms do you have?
Aftermarket rockers will hit OEM valve covers.
I would pull valve cover and take a look for something not right. Try to find the noisy lifter.
Is the noise on only one side?
Has it always done this?

Longshot... doubt it, but piston slap or exhaust leak. Does not sound like it though.