is the engine torque-ing over under heavy load only , and allowing say, part
of the exhaust system ..
or some other part , to contact the frame of the van  ?  and thus transmit
engine vibration directly to frame/body briefly .

I had one inline four cylinder engine car once..
an exhaust shop...doing dorkey work as they often do....mounted the tail
pipe so there was just one rubber gromet around a large sheet metal screw
thing...like lag bolt almost ..
right at 3,900 to 4,100 rpm , regardless of load in this one ..it would buzz
like crazy.

inline fours usually have a narrow buzzy period around 70 % of max rpm .
though yours sounds more load related..
as though the engine was torueing over and allowing something to hit.

scott
www.turbovans.com

----- Original Message -----
From: "Don Hanson" <dhanson928@GMAIL.COM>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Friday, January 14, 2011 10:13 AM
Subject: Odd sound from rear end...(not Friday and no smart 'subject'
remarks please)

>  I have an 84 with a 5sp diesel tranny and an inline VW gas motor..have
> had
> for about 3 yrs now..with this sound coming from the drive line,
> occasionally,  ever since I got the van...I have done a lot of work
> involving the driveline during my ownership.  I still hear this sound
> every
> so often, without any perceivable change in the way it presents itself or
> in
> the sound itself...at least nothing I can detect.  Here's what happens...
>
>   When I have let the clutch out totally and then accelerate strongly or
> accelerate up a steep pitch from a slow slow speed, in either 1st gear (a
> semi crawler gear that I rarely use) or in my second gear (my normal
> starting gear)  I get a sound like when you drive over a grated bridge
> deck,
> or  a vibration like when you encounter those sound strips they sometimes
> inscribe in some highways...  Kind of a gnarly buzzy sound that only comes
> with a big load at low speeds and low RPMs...So if I punch it in 2nd gear
> at
> 15+ mph...smooth acceleration only...But if I am on a sharp incline and
> going say..2-5mph and hit it,  I get that 'bridge-deck buzz' until I gain
> speed or lessen the acceleration.  It does not seem to be
> clutch-related...It happens after the clutch is fully engaged, or even if
> I
> decelerate in gear without using the clutch and then give it the pedal...
>
>   I recently replaced my engine, new clutch, throw out bearing, pilot
> bearing and had the inner CV joints out a couple of times.  They looked
> good
> with plenty of grease and no damage to the boots.   The sound did not
> change
> after having all that apart' multiple times.  I did check and re-grease
> the
> outer CV joints about 2 yrs ago, but I have not had those apart since..At
> that time, they looked OK, however, I have no real way to judge that other
> than what I have read.
>
>  I am thinking maybe I should simply do a couple of those axle/cv kits and
> see if that stops that noise...which I suspect might be the outer CV
> joints.
>
> I am thinking 'not tranny' because the sound is unaltered by gear
> selection
> and there are no alarming sounds nor did anything bad come out during any
> of
> my tranny fluid changes. No problems with gear changes, either.
>
>  Do my outer CV joints have the capacity to cause this sound?  Am I on the
> right track, do you think?  Thanks in advance..
>
>  Don Hanson