Thank you for your replies!
Last time I heard it was over french airspace. I usually here it maybe once a week. It sounds like a computer modem signal almost and goes on for a short while, maybe 5 seconds. It doesn't sound anything like any noise a human beeing is doing when transmitting/receiving. It comes on usually when nobody is talking on frequency, and everybody is listening..
has no-one ever heard this? I know it is not coming from the aircraft I have been flying as I have heard it in several different aircraft..
thank you,
EoP