I've also been frustrated by such noises on my '87 Westy. One source for me has been the exhaust pipe hitting the engine mount - the '2 to 1' pipe heading towards the CAT passes under the rear engine mount, and on my van, the gap is just a few mm. This was especially noticeable at first when starting the engine (a few knocks could be heard). However, despite two visits to the exhaust installer, and replacement of the exhaust component with a different one (perhaps a little larger gap?) there is still noise at low speeds over rough roads. My mechanic apparently does not hear it and suggested the noise must be coming from somewhere else (his suggestion - the float in the propane tank). I'm not convinced, and will follow up other listees suggestions (spring pads etc.)
Soundproofing of the floor areas has helped....

Mike
'87 Westy