95 SC Riviera Engine Tap after Rebuild

I have a 95 Riviera supercharged which needed a top end rebuild due to the previous owner overheating the engine and blowing the head gasket. Know the owner and couldn't pass up the car for $300 when I could do the labor myself. Rebuilt the engine and now I am getting a knocking that seems to be coming from the front valve cover. I have read a few threads which imply this could be due to the harmonic balancer, but I do not believe so because we tried starting the engine without the valve cover on (yes I know it squirts oil everywhere) and the tap/knock was no longer audible. Is it possible that when the heads were milled 10/1000", that it was enough to push the push rods through the head enough for the valves to tap on the cover? Idk if this is possible or if you can mill push rods down or if they make shorter ones for this. If you have any idea what it is or if what I suspect is the problem is even plausible that would be a great help.

PS. Have been getting poor milage since rebuild 16-17mpg and it was getting 21-22 before head gasket fiasco. Could valves be opening too far due to push rods and feeding it too much gas? Help plz (runs great otherwise)

Sounds like a rocker arm hitting the valve cover. Milling the heads is not enough to hit. Don't think adjustment would do that either but it might be the milage problem. Take the cover off and see where it is hitting. It should leave a mark.

didn't see anything the first time I checked, but I have driven 1000 miles or so since so I will recheck for where it is hitting. What should I do about it tapping, what can I do to resolve the problem. could this also cause the engine to burn oil very slowly, like the valves opening too much and letting oil into the cylinders?

No the tapping will not cause oil burning. If you get smoke on acceleration it is piston rings. If on deacceleration it is valve seals.