Got the same problem with mine AMK engine, but garage i took it to said it was oil pump and oil pick up pipe gauge blocked, had it done, picked it up, seemed all done , no noises on start up, got it home after long run, left it, for about an hour, started it,
problems back, i was told it could be timing chain tensioner, but the garage opted to go for oil pump, what a waste.
can someone confirm to me, that if it is timing chain tensioner, that i will be ok just running as long as i keep changing oil, for i was told if i did not have it done, it would be the same as having your cambelt snapping, engine finished