2002 Chrysler Town and Country growling noise in rear

My 2002 Town and Country makes an annoying low growling noise (sort of like a muffeled motorboat noise or like marbles rolling in a tin can)

it seems to come from the rear like maybe the tail pipe

it is most noticeable sitting at a stop light - if I shift into park it goes away

when driving at speed it is not noticeable but becomes quite annoying when sitting at a stop light

I have tried 3 times at Chrysler dealers but they say they can't duplicate it