I have to count myself extremely lucky with my Westy. The previous
owner HATED the rattling and noise and did a LOT of work to eliminate
them (and quite sucessfully I believe.)

So, here are some of what was done...

Sound insulation (NASA style foil with deadening material attached) was
put in the bottom of cabinets and walls. Under the seats, under the
mattress in the rear. Under the rear bench.Also between the metal drain
pan and the top of the stove.

Foam strips where added where the rear table and swing so that the rear
table didn't rattle. Foam strips were added where the popup arms come
down to the roof so that they don't rattle.

The jack and tool bag was put under the rear bench.

All in all, the quietness of my Westy compared to my 7 passenger van is
little short of amazing.