Back in the day many FM sources transmitted a second carrier with commercial "Elevator Music". This sub carrier transmission could often be heard as a high frequency "twittering" when the receiver was set to stereo. It was more pronounced with receivers deploying slope detection than with true FM discriminators.
Anyway it was not uncommon to have a high cut on pre's to make that sub carrier less irritating.
Anyway it was not uncommon to have a high cut on pre's to make that sub carrier less irritating.