Took Otto to the mechanic (an honest-to-God VW lover) and he deduced the
following:

He thinks the oil pump is a bit weak, and that the combo of our steeply
inclined parking area & the 20-50 oil cause the knock.  He suggested running
10-40 until the pump gets replaced.  He thinks I can hear the knock so well
because of a huge hole on the top rear (!) of the muffler.  :P

Whew.  I don't think we'll switch oil right away, but I am going to park on
the flat section of the drive.

Thanks one and all!

Mike Wright
68 Camper (Frau Blucher)
79 Bus (Otto)