Shaking/surging in reverse high idle?

[spellchek]

I've noticed the last two mornings when backing out of the driveway, if the idle hasn't dropped yet and I try to immediately back out really slow, I get this noticeable shaking/vibrating/surging feeling from the back. It goes away once the idle drops to normal. No issues in drive. I assume normal behavior but wanted to see if others had noticed this too?

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[Tuna]

I noticed something like this from time to time when I drove backwards into my garage, but since the floor is paved, I had previously attributed it to this circumstance.