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Noise after long distance driving

I have a Honda Odyssey LX minivan 2005/ with just over 100, 000 miles
On long distance driving ie 200miles or more the engine makes a groaning noise and when car is placed in Park the noise disappears and the mechanics at Honda / private mechanic and Sears mechanic could not reproduce the noise so they could not fix it. They reassured me there are no belts loose and the vehicle has been maintained at its regular intervals and recently had the timing belt changed.
Can anyone help me figure what this is and how dangerous is it?

Has anyone checked the motor mounts and the transmission mounts?
This is a known issue on Odysseys of that vintage.

How often has the transmission fluid been changed?