Just picked up a boat and that supposedly has low hours on head rebuilds plus all new manifolds and risers. Oil was very dark and thick before I tried starting it so first thing I did was change oil. Has fresh mobile 1 15w50 and a wix filter. Fired right up and runs fine but has a knocking noise coming from the top end that goes away above 1500 rpms. 40psi oil at idle and climbs to above 60 above 2500 rpms. Also , noise dosnt start until after engine is warm. Any ideas ?

Hope something wasn't trying to be hidden

Tap lifter, knock rod

Wouldnt either one still be present at 1500 rpms also ?
No noise at all above 1200 rpms . I read a different post about a guy who had a motor sit for a couple years and it make a noise similar to rod knock for about the first 45 mins of running , then went away.
Also read another thread about a guy who had very similar issue as mine and he had 2 different shops look at it and diag it as something different and never did get rid of the noise. Even after pulling motor to ck flex plate bolts, had the many other tests run and all good. He finally sold the boat.

Just a WAG, but cold oil, its thicker so lifter pumps up and for the most part stays there.

Oil warms up, thins out and lifter starts ticking.

RPMs come up, pressure increases and keeps lifter up

But you said it was a knock and not a tick

That old a motor, I would run dino oil with more zinc, but that's just me probably

Lifters collapsing at low rpm..?