Is a dimmer noisy when it's turned OFF?

Some dimmers do in fact make noise that's clearly audible in some systems. We went through 3 or 4 dimmers before stumbling on one that didn't make noise in ours.

Same with transformers (as for 12V lighting systems).

To assert that the phenomenon doesn't exist based on a single experience is no more valid than if I asserted that the phenomenon must always exist based on my single experience (which of course I don't). The varying experiences posted on this thread evidence the reality, which is YMMV.

I think the poster meant do dimmers make noise when switched off. They shouldn't make any electrical noise when switched off because the SCR circuitry should be depowered. Now, if the dimmer is only turned down to minimum and not actually switched off, that may be a different matter. The SCR circuitry would still be powered and might generate some RF noise. The typical household dimmer is a low cost appliance using a switching device that reduces the average voltage to the light bulb. That's why they do not usually work with fluorescent or LED type bulbs because they need a minimum voltage level to turn on. They generate gobs of RF noise just like when you break/make an electrical circuit. My old ARC SP-6b preamp that I had a couple of decades ago was sensitive to dimmer noise. I banned all dimmers from our house back then. Simple solution.

Oh, and also, you can dampen out the RF noise using an R/C circuit. (Resistor/Capacitor) that is the solution used on DC motors. DC motors use brushes that are rapidly making/breaking the electrical connection. A choke coil and capacitor kill the noise. All the DC motors in your car have that so your radio doesn't buzz and also not to interfere with other electronics.

I have DC hum in my preamp. I've had it in 3 preamps - all hummed (see this post preamp dc hum). I have a bunch of dimmers throughout the house and part of my trouble-shooting was to turn off every circuit at the main panel except my dedicated 20 amp main system line and still got DC hum in my preamp - grrr. Didn't hear it at one Seattle repair shop when they checked the preamp out. I got new transformer spec'ed similar to the one from the factory but have not tried swapping them yet. I also have a PS Audio Humbuster III on just the preamp and it helps, a little. Guess I will have to do the transformer swap sometime soon or move to another neighborhood (I live in one with a lot of high tech families in Redmond, WA).

My high gain Jasmine LP 2 mk II does not hum at all on the same circuit.

Another issue with dimmers that I have experienced specifically with LEDs is mechanical noise.

I have recently installed three dimmers, two for use with LED pot lights and one for ceiling lamp that uses standard style bulbs, but still LED.

I installed the ceiling lamp dimmer first, using a Levitron brand dimmer and Sylvania brand bulbs. These bulbs buzzed, despite being labelled dimmable. The buzz was very audible and coming from bulb as well as the dimmer.

In the dimmer's literature it stated that it only worked for specific models of bulbs. I had on hand a Phillips bulb that was listed for use with dimmer. I swapped bulbs and voila no buzz from either the bulb or dimmer.

I was lucky with the pot lights, these weren't listed but only cause a buzz on occasion and hardly noticeably. I don't know the brand but are likely Chinese.

All this to say that dimmer noise is dependent at least in part to the type, model and brand of bulb used.

Also I have not heard any dimmer induced noise in my system since installing the dimmers.

Finally, to answer the OP, when switched off all noise stopped.