I took my glove compartment off (only 4 screws) and took a look at things. I don't know about everyone's situation, but there are metal clips holding the plastic dash panel that covers the airbag, and the fit isn't the tightest thing on our cars. Wrapping any type of insulating tape between the clips could help. I haven't done this yet, but when I'm driving and I hear the sound, all I have to do is place my hand on the airbag cover panel, and it goes away. So I'm optimistic that placing insulating material such as tape, as bigcat suggested, would hopefully do the trick.
It's too bad so many of us have this problem, but it's not exactly a mechanical defect that Ford technicians have had the opportunity to isolate & provide a solution. Hopefully this forum is enough for us to solve this little 'annoyance'.