I just had a similar experience with my '91 GL. It turned out to be the CV joints in the rear. There was no grease (or whatever lubricant is used) left in the boots. This noise was also more prominent at low speeds and sounded like someone hitting the wheelwell with a hammer. Very alarming noise indeed, and it seemed to come and go for a few months until it got really bad. Hope this helps!

Best-
Barry, Santa Cruz
'91 GL
hopefully soon to be a Syncro Westy owner too

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