Periodic noise


My system picks up noise every eight minutes. There is a kind of scratching mid-register ("ta-ra-ta-ta") sound that last for ten or fifteen seconds. It is similar to descriptions of "GSM buzz" but it happens regularly and regardless of whether our mobile phones are on or off. It happens when the analog source is selected, not other sources (or not obvious, in my system). It is very clear when the volume is mid-high, with no music playing. It is not from the cartridge and the tonearm cable, since it happens even when I disconnect them from the phono stage. When I disconnect the phono stage, however, I don't hear it. So it seems to be picked up from the phono stage itself. It happens across different phono stages. Shielding the cables with ferrite does not help much. It seems like a very strong signal.

Anyone recognizes this problem?
o_holter
Thanks, Al. The transmitter was close to 8 METERS from the system, and yet the speakers made noise like mad. I am not in the "very worried about exposure" camp, but it was quite special. It is the pulse that matters (15-20 seconds per 8:20 minutes). If the specs only give the average radiation, they won't tell the full story. Thank you for your input, I will investigate some more. 
@almarg @o_holter 

The OP didn't say how long the interfering device had been in place or how long the interference (noise) had been perceptible. 

So my question is: Could this noise be "a canary in a coal mine?"

In other words, if the devices had been in place for awhile but the noise only recently began to be heard, would it be an indication of something failing (a noise shunting capacitor perhaps?) in the signal chain?

Not that I want to turn over too many rocks but I am curious....
Good question, dbtom2, but I guess the answer is "no" since Mr. H stated in the OP that the issue occurred with multiple phono stages, and only when a phono stage was connected and selected.

Regards,
-- Al
 
Thank you kindly, Ptss!

If anyone is curious as to what led me to suggest the security system possibility, I have no particular knowledge of such things, and I don't have one in my house.  What I did was to first Google one or two terms I thought might provide some clues, which turned out to not be helpful.  I then Googled "every 8 minutes" (with the quotes) together with the word "signal."  That led right to the security system device I linked to in my first post above, that is described as transmitting a signal every 8 minutes.

Regards,
-- Al