The mystery of "The 20", a Seattle FM radio station
with a playlist of 20 songs in an endless loop.
Nathan Barnes has
all tunes from 1958 (Eddie Cochran's Summertime Blues)
to 1977 (The Commodores w/Lionel Ritchie, Easy Like Sunday Morning).
The approximate transmitter location is not yet
determined, but one listener has a
with 4 channels of RTL-SDR which should readily help direction finding.
The more general problem of locating a rogue transmitter
is "fox hunting", and one tool for that is using a directional
antenna with a null that helps you zero in on which way
the signal is coming from.