What could this be, valves?

Ok guys i have problem, a new one. I just dropped a 6bolt in my 2G andi have this loud squeak from the engine, i think its the engine, pretty sure but what could it be, valves? also i had something in that cylider before i started it up. that little tid bit of info could be helpfull. what do you think?

im and idiot with engines but id check the small stuff first.. belt tensions, pulley bearings and what not.. or maybe theres just a mouse living in there somewere :)