if it was the lifters it would tap contanly if one had collapsed, it sounds like big ends to me mine went on friday. when the big end bearings go the knocking is caused from 'piston slap' so there would be no more noise at the sump than there would be at the head. does the knoking start when you rev it and when the revs stabalise does it go away then come back when you decease the revs?