The gas furnace has an electric igniter. When the thermostat triggers it, it makes the rapid sparking noise just like your basic stovetop and other furnace igniters. But, if the stereo is on, the clicking comes out of the speakers. I checked grounding at both the stereo and furnace. I moved the stereo to the opposite leg of my service. I tried two fancy audiophile filters (thankfully money-back guarantee!)
I've run out of ideas apart from an expensive isolation transformer. What have I missed?
There is a lot of RF noise coming from any spark lighter like this. It may just be airborne and not coming from anything wired into the set. The first thing you need to do is figure out where it is getting injected into the signal path. I would disconnect the speakers and try a headset with a shielded lead to eliminate the idea it is coming directly into the speakers, although that is unlikely. Does it seem to favor any particular input? You could make up terminator plugs for any unused ones. If not you may just have it being "detected:" by a diode in the amp and I am not sure what you do about that other than better shielding around the amp. I doubt an isolation transformer will do anything. If this is a fairly new amp, it has an inverter power supply.
What are the conditions at the stereo? AM, FM, Tape Mon, Aux, Video??? Does the volume setting impact the noise volume? I would try installing some low pass filtering as close to the igniter as possible. Joe
May be a silly idea, but are both circuits on RCD? I suffered a similar problem quite a lot of years ago and gave up in frustration trying to solve it, eventually putting it down to my audio system being "home brewed". In my case I couldn't attribute the source to one item. Eventually I updated my panel to an RCD one, incidentally discovering a neutral earth short half way round my ring main. Doh! It was a neutral wire knicked by the faceplate screw. Once fixed, the audio system problem disappeared. I believe that the currents circulating between the short and power source behaved like an hearing aid loop system, radiating at low frequency around the house and picked up by the stereo.