I have a 2011 F150 FX4 with the traction control. When the tires spin (like taking off on a hill in the rain - that's when i noticed it) and I let off to let everything settle down, I hear a rattling kind of noise coming from the middle dash area, maybe on the engine compartment side of the firewall. It sounds like a rubber ball in a box being jostled around.

Any idea what this might be?

 

Its one of the monkeys that Ford uses in tight places for assembly. Apparently yours did not get out. When I had my Expedition I tried to get an aftermarket monkey to change coils and plugs but had no luck.

 

If it's a 2011, do you have it in A4WD? You may be hearing some of the front driveline components actuating.

 

I am thinking its the vacuum valves actuating.