Octavia 1.6 engine problem

i've just had my head gasket done and picked the car up on friday,

all was fine.

done the usual of taking it easy, never pushed above 4000rpm and have covered 100 miles with no problems whatsoever, decided to check the oil and water levels all seemed fine til i started the car back up..

got a knocking/rattling sound from the top end of the engine, luckily this happened at my mates garage and he carries out all work on my car but this issue is puzzling him, what the problem seems to be is the jets or whatever spray oil onto the camshaft aren't doing there job properly, it was as if one was blocked and at higher revs the other 3 seemed to be providing oil enough to quieten it down. he managed to get all 4 back to working but its still knocking/rattling and he's unsure as to whats causing it.

has anyone had this kind of problem before or any advice.

at the moment our only thought is to run some engine flush through and give it another oil change as theres probably still a lot of gunge in the engine from the gasket going.