Loud tapping noise after complete overhaul of the engine


I have a 1970 3500S NADA and was forced to do a complete engine overhaul.

Despite this, there is still a a loud tapping noise, which seems to come from the top end. The engine runs good and really smooth, has drive 50 kms but the noise stays.

Engine parts renewed:

Rebore and new offset pistons + .020.
Crankshaft ground .020.
New camshaft and followers. Cam followers adjusted 1 mm.
New rockers and rocker shafts.
New exhaust valves.
Valve clearance in overlap checked.
New chain and chainwheels.
All clearances checked and double checked and found within specs.

It is a 3.5 engine and was checked for loose liners. The engine is not original unfortunately and is from an SD1.

The noise becomes less at revs above 2000rpm.

Loose valve seats , valve guide?

Noise is also in a cold engine but becomes louder as temperature rises.

Oil pressure is 2.5 bar at tick ,over hot engine.

Does anyone have some suggestions what could cause this?

In advance, many thanks for your time!
I have no technical knowledge whatsoever, but do recall from decades ago that if the hydraulic lifters are drained of oil this can happen. Not sure if they refill during normal operation or whether they need to be removed and refilled. But please do not take this as authoritative.
Oil pressure seems a bit high and how have you adjusted the cam followers?

Did it make this noise prior to the rebuild?
Do you mean adjusting the tappet preload when you say "cam followers adjusted 1 mm"?
I ask because I'm wondering if perhaps you didn't allow for the rocker ratio and put 1mm spacers under the rocker shaft pedestals without realising this would lift the rocker cups by 1.6mm?
Have you by any chance used heavy duty push rods?
These can clash with the cylinder head unless the holes are enlarged to suit.