2002 Ford Focus SE-automatic

This problem usually seems to occur only when the engine has not been on very long, so when I first pull out of a parking space in the morning or haven’t driven in a few days.

The problem: When I put my car into reverse, the audible volume of the engine’s idle increases two-three fold. The interior of the car also shakes a bit. Once I move into drive or park, the sound goes back to normal.

Is this a transmission problem? What are other possible causes for this strange increase in engine volume? I do not have a tachometer on my dash (thanks FORD!!!) so I can’t tell if the RPMs are increasing, but it doesn’t sound like the same increase that happens when I accelerate. It literally sounds like my car is just idling really really loudly, and as I said the sound quiets as soon as I take the car out of reverse.

Any advice is welcome. Thanks!!