I took off the airbag today and one of the horn ring screws was a little loose, and that's all it took to make the noise. That was a simple fix!
On mine, at least, the airbag-holding screws, and the horn screws, are ALL T-25; didn't have to use my newly-acquired T-30 screwdriver at all.
Thanks to all for helping!
PS to anyone going to do this: please do follow the recommended procedure for airbag safety. The manual says:
1) Disconnect negative battery terminal; then
2) Disconnect red SRS test connector under passenger's footwell
before you take off the airbag.