I have seen flushing oil and the additives destroy some engines in the past, personally i would never use it
Just regular oil changes using good quality oil is all you need to do.
Sometimes noisy hydraulic tappets sort themselves out as uphill racer has found.
At one time if you fitted new ones, it was best to leave them in a tray of oil overnight to prime them.
Glad its sorted though
Thanks, Tony. Ordinarily I just use good quality oil and regular changes in the rally car and hillclimber, wouldn't ever bother with flushing oil. In this case though, as per the OP, this was a head that came to me with unknown provenance - and apart from the chance of some polystyrene in the oilways ** - I did suspect it had been sitting for a little while, so any heavier components of the oil in it may have started to thicken/solidify. So hopefully running clean oil through it would eventually clear the tappets and allow them to pump up properly.
** the head came in a big cardboard box, packed out with chunks of expanded polystyrene. No bag or any other wrapping. The seller hadn't thought that the polystyrene would shed individual beads while handled in transit, so when I took it out the cam chamber was full of loose beads! I fished out everything I could find but was then worrying about how many beads were still stuck somewhere... I've run it up a few times now to get it all the way up to temperature, in the hope that the combination of heat and oil will break any remaining polystyrene down before it does any damage