Whenever I play a radio station on my pip-boy, there is a rapid "ticking", or a faint "clicking" in the playback.

I notice it most with Existence 2.0, but it happens with all the stations (I have replaced the tracks played on GNR with my own music, but Agatha's station and Enclave station are vanilla... and all have the issue).

I've had this problem as long as I can remember, and have spent a lot of time messing around with the setting in the Control Panel.

I have a Creative Sound Blaster X-Fi Titanium Fatal1ty Pro sound card.
Fallout 3 is the only application that has any sort of sound issue at all.

Has anyone heard of something like this?
Anyone have any idea what may be causing it?
Any suggestions on something (config/settings wise) I could tweak, tinker with, or otherwise molest to fix it?

I have never experienced any sound related issues with the game. I would have to say it has something to do with your sound card. Try using stock sound on your motherboard (if you have it) and see if the issue is still there.