Not sure if it was mentioned yet, I suggest checking each rear shock by
grabbing and trying to twist it.  You may hear a what your looking for...

I had a rear shock with loose top bolt make the sort of sound you
describe...

Jim
Wilmington, NC

On Sat, Apr 2, 2011 at 3:50 PM, Rocket J Squirrel <camping.elliott@gmail.com
> wrote:

> 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?
>