I've been having the same issue since I bought my car. Also speed-related. Dealers have acknowledged the rattle but, to date, no fixes.
One recent development... for the longest time, I thought it was coming from the pax door area. However, on a long (500 mi) trip last weekend, my wife said she's almost positive it's coming from under the center console. Since it's speed-related, I'm 99.9% sure it's outside the car and I'm now thinking it's probably the heat shield under that area. I plan to ask the dealer to take a look down there next time the car's in.
2012 Focus Ti Hatch | Ingot Silver | 401A, Nav, Winter Pkg, Moon Roof, Parking Tech. Pkg
Build Date: 12/2/2011 | New APIM: 4/4/2012
MFT: Factory v2.11, 3/14/12 USB/Web > v3.0.2, 9/24/12 USB/Web > v3.2.2, 1/24/13 USB/Web > v3.5.1