07 4.8i rocks while in D at idle/stop light

[mr_robot]

Trying to address all drive ability issue before I tackle any other issues and stumped on this problem now.

The best way to describe this is as if you were trying to accelerate a little while in D while holding down your brakes. The car rocks back and forth enough to be noticeable from someone looking from the outside or the if you have a water bottle in the cup holder you can see it move. But it’s very slight movement.

It’s not a vibration or misfire, problem goes away if I put in N at red lights.

I’ve been driving it to see if it triggers a MIL/CEL but nothing for the past few days.

Checked my PCV/CCV on both sides and no milky residue and both diaphragms were good.

I do see my rpm fluctuate up and down from the 500 rpms mark on the tach, maybe 10-20 rpms up/down, maybe a vacuum leak some where? Any thoughts on where I should check next?

It doesn't happen all the time which is frustrating because when trying to diagnose or show someone it doesn't happen.

Thanks in advanced fellas.