Has the pan been off for any reason while you did the head?
It sounds like a crank lobe frequency to me. I had a similar but slightly lower toned knock after my reassembly. I wound up taking my oil pan off close to a dozen times trying to locate the 'knock'. I pulled things apart and checked and rechecked but couldn't find the origin of the knock.
I finally set up my video camera and recorded the engine turning over with the pan off. Then I checked the sound track in my software editing program and saw where the knock occurred, then I could see it was at the bottom of the #3 (or is that #4?) lobe and the only thing possible was that return line. It was just barely missing the line when I would turn and check by rotating the crack by hand but once the centrifugal forces came into play it no longer missed.
