when i first bought my car,the tappets would start to rattle on tickover after a few minutes,when it was revved it was fine.the oil light never came on.Eventually i decided to take the sump off and discovered that there was no gasket between the oil pick up pipe and the oil pump,also where it had been overtightened,it had bowed the flange,and therefore was sucking it air,when it was revving it was not really a problem,but on tick over it became apparent.It was a simple fix,it cured the tappet problem,but unfortunately 500 miles later i could hear one of the ends knocking,it was only slight,but a rebuild was due.