> It turned out to be the starter motor spinning.  I put my hand on it
>                                and could feel the thing spinning
>                                away.  It was not engaged

Dear John,

The suggestions offered by Mark and others are basically sound. Another
possibility is this (which happened to me):

The solenoid might be fine, but something in the linkage is misaligned so
it won't always engage the starter.

If you need to get going, but this happens repeatedly, put the bus
in gear and rock it back and forth a few times. This will move the
flywheel a little bit (and mayby other internals). Worked for
me anyway when I had the same problem.

If you are lucky maybe your starter is simply loose or misaligned.
Try to reseat it properly.

If the problem is with the linkage inside the starter you obvioulsy
have to take it out and take it apart. You might as well do a thourogh
overhaul then (will only cost you about $20 in new bearings and brushes
and take about 1/2h). See the detailed instructions and parts list I
posted (fall 1998). You can also have a shop do it for maybe $150-200.

BTW: Check if the spinning you notice when the key is relly due
to the started being powered. It can take it a minute or two just
to spin down from having run up the rpms unloaded. (These are series
motors, and spin extremely fast under zero torque conditions, remember
your electrical power systems course?)

Cheers,
Martin