Strange Noise from PrimaLuna DiaLogue One

I have been the happy owner of a Prima Luna DiaLogue One (the *non* "premium", original DiaLogue) for about 8 years now. I love the thing to death.

My system consists of Sonus Faber Elipsa SE (or "Red") speakers, and Oppo 105, and Nordost cables. Most of my listening is via CD's or my own custom-made server (still using the DAC in the Oppo).

Here is the problem I have been experiencing as of late:  

I am getting a kind of noise coming from the tweeters of when using the PrimaLuna. The noise the same in both speakers and even happens when the amp is on "mute".

The best way I could describe this noise is that it sounds like "interference". A sound similar to what one normally experiences when a cell phone is placed close to a speaker and there is an incoming text message. But in this case, the intermittently "beeping" noise is almost constant. Impossible to hear when playing music, but it can get distracting during silent passages.

I am baffled by this, because the PrimaLuna has been exceptionally dependable, reliable as a rock, and dead-silent as of today.

I tried changing tubes (all of them), turning off my Wi-Fi router (not that it makes a difference, since I live in a condo and there are literally 10 other signals in the air 24/7), I made sure nothing was charging in the house, we do not even have dimmers anywhere... but no luck...

Today, my dealer let me borrow a McIntosh integrated amplifier. I have been curious to try out a solid-state amp, as I believe my speakers (not the last word in efficiency) would not mind the extra juice... With the solid-state amp, the noise is gone! Baffling!

Even if I end up liking the Mc, I will not part with the PrimaLuna, because I love it so much. But I would like to find out why I am experiencing this issue... Any theories as to why this may be would be appreciated.

Thank you all in advance!