At stop light, with brake on, slight drop in RPM
 

When sitting a stoplight with the brake on, there is a very slight drop RPM (-100?) and a bit of a very slight, soft, "shudder", sort of like a miss, but not really.. Almost imperceptible, but it's there. Take foot off the brake and it goes away. Put foot back on the brake and it returns. It seems to happen independent of whether the A/C is on or off.

Any idea about what causes it?

2012 Copper S, 6-speed. .