I was going down the m4 and the rad cracked it started to make a loud tapping at about 110mph so i slowed and took it easy got it home and thought the tapping was the bearings so i changed them all (main and big) still tapping so ive been changing stuff ever since,still same noise ive tried new cam,new followers ,rockers and springs,new head,new oil pump,new dizzy ive checked the pistons and rings they seem fine the oil preasure is low at about 12psi when warm. But i have no noise and oil preasure is fine under load. Does any one have any ideas?