'Roaring' isn't very descriptive. With bearings, you would hear a
'whoop, whoop' rotating sound from one wheel specifically and probably
feel some vibration. A transmission would probably be more of a whine.
Could be something hanging down underneath, in the airstream.

It COULD be the exhaust. When you're in park, you're idling below 1000
rpm. When you're driving, you're almost always going to be above 1000
rpm, so the sound and resonance would be different.

Dan D
'07 Ody EX
Central NJ USA

Invite your friend to your car, ask him to sit in the back and stick his/her
head through the window to get better directional hearing. This way you can
tell where the roaring noise is coming from.

Inspect undercariage of the car and check for loose items like exhaust
covers, etc.