[cadillac_al]

I don't recognize that noise. I thought it got louder near the trans but you already ruled that out. I think you are on the right track with distributor/oil pump/ rear main bearing area. Some distributors don't seem to drop low enough on the intake manifold. I have done the 2 or 3 gasket trick to take up the slack. I'm wondering if your distributor is clamped down too tight and squeezing the oil pump.