Apologies in advance for the vagueness of this explanation, but it is a
strange problem...

Recently I have been experiencing a very loud CLANK (or BANG, or whatever,
a LOUD noise) coming from the rear of my 87 syncro westie.

The noise only sounds off once, and it happens only occasionally, and
seemingly always while turning.  It is not the 'Clank clank clank' of a CV
joint, as I said, I only get one loud clank and its back to normal.

It definitely feels like there is something moving back there... whatever
is making the noise makes the van slightly move, it feels like I am going
to lose control briefly, and then everything is back to normal.

It appears to happen when turning in either direction, although I�m not
entirely sure of this.

It is NOT something moving inside the van.  Sounds like it is underneath
either in the engine or between the rear wheels.

I have gone underneath and can't see anything obvious.

Yesterday's experience was enough to convince me that this probably
turning into a safety issue.

Probably not related, but I have noticed a decrease in gas mileage (-1+
MPG) starting around the same time as the clanking.

Any ideas at all?

TIA,

Jeff