issue with revit on Parrallels 10

Good Morning:
I am running Revit 2015 and 1016 on Windows 7 Pro & Parallels 10 in an I Mac Retina 5K. It has a 4GHz Intel Core 7 with 16GB of RAM. The Mac is runing Yosemite 10.10.4.
Revit works wonderfully and I only have one annoying issue that Parallels support doesn't seem able to resolve: Every so often and apparently at random while in Cherence mode a small gray rectangle appears on screen on top of the window. The only way to get rod of it is minimizing and reopening the window or exit Cherence.
The only software I run in WIndows is Revit and Avast antivirus.

I haven't contacted Autodesk support since it seems to be a Parallels or MAc issue.
Has anyone experienced this and found a solution?
I would appreciate any comments on this.
Thank you
Carlos Ostrej