In terms of tinnitus, is there some biological activity going on in silence that isn't happening in ambient noise?
Yes.
Your auditory system exists to detect external sounds. And in a very primitive sense (like from cave-man days) it exists to protect you from the enemies that might only be detected by virtue of the sounds they make. So in silence, your auditory system does what it is supposed to do and tries very hard to hear what's out there. And since nothing is out there, it ramps up your own internal sounds (i.e., your tinnitus) to feed its need to hear.
With folks who are largely habituated to their tinnitus, that just does not happen - because the habituation process dampens the autonomic response as far as tinnitus is concerned.
stephen nagler