Caravan rumbles when it rains

This wierd! :frown: There's an embarassing loud rumbling noise from the rear-but only when the road is wet. Tail pipe isolator has been replaced. :banghead: Didn't help. What could it be? Could it be the muffler? Vehicle is a 2005 Gr. caravan with stow and go. :uhoh:
Texana

Here is something to think about, in these vans the rear wheels have a wheel bearing/hub assembly. If the seals in it wear then when it rains then water can get in the bearing and cause some rumbling noise. I hope this helps. You have to replace the whole assembly and is not too cheap at about $150.00.