You need to have the Diff Fluid Service Bulletin Performed as stated above.
I went through a similar situation after installing Springs/spacers thinking i was rubbing. I switch spacers, then realized it wasnt that and had the e-brake cable looked at thinking that could be getting caught up some how..
The way you describe it is spot on. It would sound like a moan, it would slow the cars speed when it occurs, Happenening at low speeds in a turn without the wheel at full lock..
Also it would be more prevelant while the diff fluid is cold.
Get the bulletin done, give it a few miles to work throughout the diff and you should be all fixed.
2008 E92 M3 (03-08-2008-first delivery)
JET BLACK on BLACK w/Carbon Leather, 19's, Tech, 6spd..