At strengths between S2 and about S4, fading up and down, was an FM transmission, consisting of a series of tones followed by an occasional announcement of "500 feet". I'd never heard this before and wasn't entirely sure what it might be. To start with, I thought perhaps it was a data from a balloon flight, but as the heights were increasing and decreasing quite rapidly, I suspected that was unlikely.
However, GB3TD sprang into life and Andy, G0BEQ popped up, testing his new Baofeng UV-3R (which sounded great!). I told Andy what I was hearing - and interestingly, about 15 miles away from me, he wasn't hearing the interference. He was able to tell me what it was though; equipment for model aircraft that reports the altitude of the model plane back to a receiver on the ground.
Oddly, after Andy and I had been speaking for a few minutes, I noticed that the transmission stopped. A coincidence perhaps, or perhaps the signal from the repeater being rather stronger than the few milliwatts from the plane had caused the altimeter to stop working whilst the repeater was active.
Not sure about the legality of these devices - although my suspicion is that they come under the heading of the low power devices and can thus be operated legally. However, if you are a model aircraft enthusiast reading this and you have one of these devices, may I ask you respectfully (genuinely) to try and use a channel that amateurs do not use. If you are in doubt - then your local amateur radio club can probably advise you which frequencies are likely to be in use local to you.
It's surprising how far a milliwatt or two of FM on 433MHz will travel!