I bought one of those auto stethescopes a couple of years bachk and they really do work well. if you don't have access to that you can hear quite a bit by pressing a big screwdriver to your ear, and to various spots on the engine.
or you can make something up from tubing etc if you want to get creative.

the auto stethescope has a rod at the end which connects to a tin plate , which helps amplify the sounds. it is surprising how much you can hear going on in there.

I have had a timing gear come loose , and it made a racket! you can change it without taking the pan off. and you can get the pan off if you jack up the engine,with the motor mounts off but you have to finesse the oilpump out first.

also if you think it might be from the valvetrain you could try adding a .010 or so shim under the valvesprings. valve float can make a clacking noise. I had this problem and it took a while to find. just don't put too much or you could make the coils bind.

Phil