After you get the pan and tray off, turn it crank side up, see if you can wiggle any of the rods side to side with your hands (you shouldn't be able to, at least not perceptibly). Wiggle the crank back and forth just a little while feeling each rod cap with your fingers. You shouldn't be able to feel anything "hitting", no discernible slack anywhere. Turn it so that one bank of cylinders is "sideways" - turn the crank so that each piston is at the top of the bore, then **GENTLY** tap it on top of the piston will a rubber mallet while feeling the other end. Again you shouldn't be able to feel any slack at all. Turn each one 180 and repeat, tapping **GENTLY** this time on the rod cap. (If you whack too hard on it in either direction, you risk "flat spotting" and ruining the bearing.) If you find one or more that seems to have some "give" in it, that's likely to be where your problem is.
Look inside the windage tray for any evidence that the crank has been hitting it.
If it's still a mystery after all that, then the next step will be to take the bottom end apart and inspect (and measure) all the bearings, also see if you've got a piston pin problem, but don't go down that road just yet until you know you have to. If you DO remove any rod caps, make double-dang sure that you mark everything first so you can put them back exactly where they came from.