At least that seems to be where it's coming from. It is sort of a low pitched whirring sound. To me it sounds like bearing noise, which it could be. Could also be pinion gear in the differential, I gather. I got under there last night to check out the U-joints. The rear one has a little slop (it shouldn't have any) so I'm planning on replacing both tonight. But from what I've read online, the U-joints would cause vibration or clunking, but not the type of noise I'm hearing. It doesn't seem to vibrate much. The noise is there regardless of acceleration/deceleration, and is there if I just put the tranny into neutral and coast. Mostly it becomes noticible at speeds over 30 mph. Doesn't change if I turn the vehicle left or right, either. So I'm guessing rear wheel bearings.
After cleaning off 1/4" of oily road grime from the differential exterior, I noticed some blue silicone sealer around the cover and bolts. So I'm thinking it's been serviced once before, at least. Perhaps just to drain the oil, although I did find an oil filler plug, and it should be easy enough to stick a tube in there and suction out the old oil for a change. I doubt that the factory used blue silicone back in '67.
I'm wondering how hard it is to find a complete rear end (junked) for one of these vans. I understand this van was loosely based on the ChevyII. Would a ChevyII or Nova rear end of the same vintage bolt right on?
I also gather that this van (G10/20) has a semi-floating rear axle which appears to be relatively easy to replace the rear wheel bearings on (as compared to the full floating rear axle). Anybody tackled that job?