Engine Knock help

[mrr1999]

Hey guys-

I just pulled the 396 out of my Camaro to repaint the engine compartment. I hoisted it out and left it hanging for a couple of hours while I painted. I lowered it back into place and when I fired it up, there was a really loud knock coming from the oil pan. Not a typical knock, more like somebody was hammening the oil pan. The motor never got bumped or slammed back in. It ran perfectly before I hosted it and a few hours later, this happened. Any ideas?

I can't see the cap on the connecting rod coming off or anything catastrophic happening. I am totally stumped. The weird part is that the engine, despite making that sound, runs perfectly smooth from idle to revving.

Thanks.