Rumble in Reverse

I happened to be moving my wife's '95 E320 today and noticed that when I put the car in reverse - with the brake pedal down - there was a slight rumble
(vibration) feeling. I don't get this vibration in forward. And the car seems to drive fine in all gears. I checked the tranny fluid - it is fine (and it had it's regular 30K service about 3K ago).
Could this simply be a vacuum leak (is there something on the intakes that gets activated in reverse that is not activated in forward), could this simply be the MAF showing its age, or does it sound like the reverse gearing is starting to go?
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