There is this very faint (presumed modem) signal I started hearing in cars back in the early 90's. I'd heard it before in earlier contexts but don't remember any details.
In the 90s and early 2000s, it was not uncommon for me to hear some modem-like sounds from various devices with speakers (PC speakers, audio system speakers, car speakers). The sounds were basically interference from nearby (as in, mere feet away at most) cellphones of the era. I assume since then that cellphones have become less leaky and/or consumer electronics have become better at filtering out such interference, because I don't really notice this phenomenon anymore.