In the days before round-the-clock radio if you tuned in before the start of the day's broadcasting you'd have heard lots of beeps and whistles. Not some alien communication from outer space but test tone transmissions. These were designed to "assist in the setting-up and checking of stereo receivers."
Here's what they sounded like (yes I really am posting a few buzzing sounds!):
You could get a BBC Engineering Information sheet to explain exactly what you'd hear:
More BBC Engineering information sheets can be found here.