Can't find oil leak.

1997 4.0 OHV 2WD XLT 4-door explorer with ~130K miles I've had a slow oil leak for awhile. Thus far I've replaced the intake manifold (upper and lower) and valve cover gaskets. I took it to a mechanic earlier this week and they diagnosed it as a leaking rear main seal. They also replaced the oil pan gasket. It still leaks oil. And now my engine has a rattle that sound either like a timing chain or the valves clattering. They had to take the transmission out in order to replace the rear main seal. I've spent $800 on repairs so far and nothing has worked. Does anyone have any ideas? Thanks.