I have seen this more than once. I would hazard a guess that you are running a flat tappet cam??
If so you probably have 1 or 2 lobes and lifters worn down. They sound like a knock.. If your
oil pressure is still decent that would indicate that the bearings and important
oil bathed stuff is OK! Pull the lifters and check those first. The contact face should be convex on all lifters.If you put 2 lifters end to end..flat face to flat face they should "rock" on the high spot in the middle of the face.
John