I just experienced the same thing with my 327 drag motor. Sounded fine, ran great, but had a slight rattle sound above 2,000 rpm and the oil pressure began to fluctuate.

Turned out I had a main bearing cap break in two and apparently the crank vibrated so violently that the other main caps are egg-shaped and will not hold a bearing shell(nearly 1/8" play between shell and cap!). Hope yours isn't like mine!

Bowedup