The fact that you say its not there when its raining tends to tell me it is probably suspension related..check for muck around the top and bottom area of the springs and rubber bushes in the shock and double check the axle bush just because it a MW and has a big heavy booty..

dont forget to check the shock as well, there could be a stone or rubbish inside the plastic sleeve that been caught.
the rear axel on the mw is a doodle to drop, i done this last year with the wifes, it may pay you to just remove the bolts on the bottom of the shocks (it will drop then given you access to the top of the springs) then clean and lub everything up with waxhol - this should stop any of those annoying rattles that come from these big beasts.
i can do this job on the floor in aboput 30 mins.
roy

This problem is very common in MWs, mine has it too and is related to the bushes.

As far I know, bushes in the MW rear suspension cannot be lubed, even if you are soaking then in oil, the oil may not get to the right places.

it is a real pain in the butt, you'll have to remove the whole axle and probably replace the brake pipes while the axle is out.
easiest way to put the bushes in is get a "reamer" and ream the axle to accept the bushes easier.