Broadcast FM Whistling
Solution: Probably single-transistor SAW oscillated TXTransmit, probably hobbyist. Data input and/or Vcc of TXTransmit seems to be discharging a capacitor which is periodically fed a charge pulse; doing that to a single transistor TXTransmit results in exactly this, a signal that gradually drifts it's frequency while lowering it's amplitude. This can be used as a tracking transmitter, compatible with USBUpper Side Band Modulation (Radio, referring to reception and modulation mode)Universal Serial Bus (Computer, referring to USB Ports and cables) mode; it could be for a lost RC plane finder for example. The lower/higher pitch tone audible at the fadeout-spot, and the louder the general volume, the closer it is.