Popping noise in my Quad ESL 63 pro monitor

Could also be a cap. Whatever it is, you must check out the circuit. If you're not experienced & qualified to do this, do NOT do anything. Rahter, get a qualified electronics person to do the checking -- there are high voltages in there!

Sometimes dust and animal hair can enter the Quad ESL 63 and cause arcing. I suggest disconnecting the speaker, letting it discharge for a half hour, then rapping the sides forcefully. I have had some success with this method over the years.

Let us know if this does not cure the popping. If the popping is disrupting your listening, or non-listening sessions, you may be discovering that it is time for a panel replacement, or complete rebuilding (not a bad idea if you want these things to continue to make sweet music for another 20 years or so).

for the music,
Michael

I'm not sure if this applies to you, but I've owned Acoustat speakers that had this problem. I redirected the way that the wiring comes out from the panels to the transformers and the problem was solved.