The thunking only occurs when the drivetrain gets slack. Like taking
your foot off the gas pedal while cruising slowly on a flat street.
Re-applying pressure on the pedal does not cause the noises. Sometimes
when the van is moving at just the right speed . . . 8 mph or so . . .
on a flat street, sort of idling along, you can hear the mystery thunker
going thunk thunk thunk. It's a heavy sound, can be felt a little bit.

Second gear seems to be perfect to set up the right relationship between
engine power and van moving resistance, allows the right amount of
slack. If that makes any sense at all, first is too "tight" and 3rd,
well, there's a lot more road noise when up in third it might be
happening but can't hear it.

If this sounds like a suspension bushing thing, then we'll look again
there. But it feels like a drivetrain thing. But the engine does not
move visibly when thunking, the transmission and diff were rebuilt and
the noise was happening before and now after the rebuild, and Gary just
checked the axles.

It baffles science.
--
Rocky J Squirrel

On Sat, 2011-04-02 at 09:34 -0700, Don Hanson wrote:

>
>
>
> On Sat, Apr 2, 2011 at 9:10 AM, Rocket J Squirrel
> <camping.elliott@gmail.com> wrote:
>
>         I took Mellow Yellow to see a new doctor . . . I mean mechanic
>         yesterday. There has been this mysterious "thunk" sound
>         whenever I'm
>         cruising at low speeds on flat surface streets and take my
>         foot off the
>         accelerator.
>
>
>   Suspension bushing(s) maybe?