1989 Vauxhall Corsa

1 bent valve new cylender head gasket new valve this has now been fixed problem now is when u start car all is fine untill car has had about 5 mins heat in it
then the oil light comes on and there is a noise coming from the rocker cover area it is a ticking noise which get louder and louder. When this car was fixed took it for a test drive all ok 10 miles in to test drive oil light came on and the ticking noise started
got back to base checked oil, oil ok let the car cool down them started the car up again no noise untill it heat up again and noise started agian ticking noise coming from rocker cover area any help please