Periodic noise

Thanks for help! No neon signs nearby, no special equipment on, in our house. The noise is regular with 8 m 30 s intervals. It comes and goes a bit, and sounds like a wireless interference, much like standard mobile phone interference, except it is regular - rather than AC interference. We do have a security system in our house - I will check, could this be the cause.

Someone is trying to tap your phone.
iPhones are less-likely to be traceable and provide the best privacy

You might want to check whether your main power board has a smart meter that monitors power usage and transmits power usage info back to supplier at regular intervals.
Alternatively the source could be from neighbours, there are so many sources for wifi these days - routers, phones, many household appliances now have wifi active.

almarg - you were right. I unplugged the main alarm system control box and disconnected the battery. Now, the noise was gone. I will get service on the system tomorrow. It is both wired and wireless.

My wife signed us up for Verizon Fios.  At one point, the Fios was going down with regularity, until I finally investigated our circuit breaker box, where they had installed an AC outlet to power their system, which is a simple plug-in.  It was easy to demonstrate that power to the plug was intermittent, by simply wiggling the plug, causing our problem with lost service.  When I opened the housing for the new AC outlet their tech had installed, I found that the wires were nearly floating free in the box, just barely in contact with the outlet, because of sloppy attention to detail.  At least it was easy to fix.  Then I noticed that the Verizon crew had tapped into my dedicated AC lines to power the new outlet they installed for Fios, thus assuring that my audio AC was constantly polluted with digital garbage. You've gotta keep an eye on these installers.