Hi all fellows who responded in good will. After 1 month of efforts, here a recap:
1. Since some time I've got a "whistling" noise when I plug cables with or without guitar(s) in any of my FOUR amps (three tube and one modeling Line6). Frequency of the noise +/-2,5 kH. I've never had that problem before.
2. To exclude the possibility of interference from a unit, from lights LED, or else, I shut down the power in my house (switching off all the breakers in the service panel) leaving only one wall socket, so absolutely everything disconnected - but the "whistling" is still present !
3. I tested by bringing one amp and a guitar to neighbors in the one and the other end of my neighborhood and the noise is present everywhere !
4. One of the neighbors (a just retired electrician) got the power supply in his place through a transformer (as a filter) and a stabiliser - still the noise was present.
5. That neighbor measured the ground and voltage in my house and they're in good limits: 10 Ohm, 238V. (My house is just 15 years old.)
6. I also tested by going to two other friends' places in different areas further away from me and... NO NOISE over there !!
7. That all suggests that my problem is NOT related to power lines or to grounding, but to an electro-magnetic emission coming from somewhere.
8. I tried passing through a DI box - the "whistling" still present !
9. I borrowed from a friend a wireless system (transmitter on the guitar and receiver on the amp) - the "whistling" still present !
10. I made an XLR mike cable "one-end-bond-shield", so the shield connected to ground from the one end only like sort of a "Faraday cage" for the cable - the "whistling" still present !
11. I was advised and assisted to make a filter with inductions and capacitors corresponding to the frequency of the noise (+/-2.5 kH). That filter was made but... gave no solution - the "whistling" noise IS always present !
12. An interesting detail, when having a guitar plugged, if I turn the Tone Control knob lower, the noise goes weaker till disappearing. So the Tone Control works like filtering it. But then my sound goes too dull which is not what I want. And that's not an acceptable situation.
I'm brought to extreme frustration, as my work is totally blocked. I guess I'll have to bring a qualified electronic tech person to possibly find a solution. Hopefully it won't be extremely expensive...