James9x19 I think there is a faint possibility that some app is misbehaving, which might be detectable by factory-resetting the phone (which would lose all of your data, so you'd need to take appropriate measures) and adding apps back one by one.
But if it's not an app, and if it's happening at unpredictable places not near powerful transmitters, then I think it's more likely that some piece of hardware is failing -- perhaps a filter, perhaps a mixer. Perhaps that part was marginal before the trip to the transmitter-dense area, and the visit pushed it over the edge (though nobody will ever know).
It's possible that somebody with a spectrum analyzer could estimate based on spurious emissions which part is going bad.
Overall, I suspect some mixture of a factory reset and a new device will be needed.