It's definately not the passengers as they moan all the time.
The sound is similar to the noise that occurs in the flare just before the stall warner goes off, but comes from the wing roots.
As Bp & Ua suggest it must be a subtle change in the aerodynamics.. Thanks for the replies. Other C182 users see if yours does it. Ours is a C182T
Ua. Yes, heavy rain knocks at least 5 knots off cruise.
D.O.