[Brentwood Bob]

The distributor should not, IMHO, be firmly tightened at the head, but the retaining set screw on the side of the head that keeps the distributor in place should be tight enough to hold the distributor. There is a notch on the bottom of the distributor casting that the set screw captures. Too much down pressure on the oil pump driver can bind the cam and cause a rapid knock that you might be having. I suspect this since you may have removed the distributor recently and disturbed it.
Bob