Originally Posted by Stretch

If the oil pressure takes some time to go to maximum after you've started the engine after it has been stopped for a while then it is likely that the oil filter return valve is duff. Oil could be draining out of the filter into the sump - it then takes a bit of time to get the filter filled when you start the engine again and the top end of the engine rattles because it is being starved of oil...

...if it is this - solution is to put on a good quality oil filter.