id say either rod bearings, camshaft chains, or doubtfully shim. If your SHO has some mileage >125K could be time to replace rod bearings. When my rod bearings were going bad I could here a slight knock. It was more noticeable if you listen near the passenger side wheel well. Id get a long screw driver and start probing it on different areas of the engine and see if you can pinpoint it. When a shim is out of spec it shouldnt make a loud sound unless you threw a shim, which is unlikely unless you just did a 60K adjustment. Regardless of your problem, i would limit trying ur SHO until you can find the problem. Of course, the valve covers are much easier to take off than the oil pan. Id start there, look at the chains and valve lash. If you cant find a problem there, more than likely its your rod bearings.
