"Steven P. McNicoll"
It's referring to "whistle stop tuning", introduced by NARCO, I
believe, in the late forties or early fifties. Back then you'd
transmit on one of four(?) crystal-selected frequencies but select the
receiver frequency on a tuner that covered both nav and voice bands.
Activating "whistle-stop tuning" turned the transmitter on at very low
power, when the tuner reached the transmitter frequency you heard a
whistle and knew you were on the right frequency.

Steven, I thought that the heterodyne whistle "stopped" when tuned to
the exact frequency. Been a long time ago though.

Bob Moore