Intermittent Overdrive O/D light flashing w/ engine making noise and power loss

What is your question?

The O/D light of my 1998 diesel Prado flashes only intermittently. Most of the time, the vehicle drives fine. But sometimes the O/D light starts flashing. When that happens, two other things happen: (1) noise starts coming from engine area (not sure exactly where.. sounds like a loud 'out of tune' diesel truck), and (2) significant power loss in low gear/start up hills. My mechanic thinks it my be (1) a blown transmission; (2) fuel pump problem; or (3) "some kind of electrical problem". Not very promising.. I live in Kenya so skilled technicians and diagnostic systems are hard to find and pricey to access. Any suggestion? Thanks Tate