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is the chassis of the PC properly grounded, the card sits firm in the slot and has a tight connection to the PC chassis ?

cheers, Tom

I was supposing all being grounded, but...I will check for the chassis and for the AC line in my room.
As for the cards, though the chassis comes with fast clips (no screws mounting), I screwed all the cards to it. It wasn't that easy. There were holes in the right places but without screw threads.
I will check them too.

It's strange. I was restless drunk, or the noise comes out from one speaker only, but I can pick it up from both the outputs using the oscilloscope probe. The same signal is present on the hot wire and on the ground; this is what I've found touching them with the only hot wire of the probe. Just, I forgot to check for amplitude differences. If there's any, however, it's quite subtle, otherwise i should had noticed it..

I've to try with headphones, and changing the channels for the two active speakers

Just checked the grounding:
The wiring in the house was updated to CEE directives some year ago and has proper grounding.
I've measured values close to 0 Ohms between any point of the chassis, included boards brackets, and any point of the ground wiring in the rooms around.
Even the ground of the RCA connectors of the Lunabox shows almost 0 Ohms with house ground wiring. I'm just wondering how I was able to pick the noise signal up from there

astroman wrote:.....the card sits firm in the slot ....?

I had the same noise having the cards installed in my previous PC....

Ok, I repeated all the tests.

The grounding is 100% OK.
The noise is present on all the outputs for both the speakers and it's on headphones too.
The noise signal can be read on the ground wire and on the hot wire with the same amplitude.

Tom! The noise disappears setting the brightness at 100%!
And the image quality is not bad with that setting.

yes, that is correct - if I lower brightness below 80 (0-100scale) I can even hear the screen with plain ear. I've set contrast fairly low (36) to avoid sunglasses in front of the thingy...
It's a cheapo Fujitsu Siemens 22", so I wouldn't complain anyway

cheers, Tom

Being the 170S absolutely not a shiny screen using flat settings, there's almost no need to compensate with contrast. Even in a very dark room, 100% brightness ain't much annoying (in the short term at least).

With my usual rig I've noise using guitars, being the 2nd screen a CRT one; but this can only be solved replacing it, I guess. At now I use to set it off during guitar sounds shaping or takes..