Hello Malthusian,
Question: if you turn down the bias, does that sizzle disappear?
May I ask how old are your panels? How long have you heard this symptom?
I am not fully able to guess exactly what might be the cause, it could be a dust particle that has found its way in, and that may disappear after time. (panels are generally self-cleaning) (rather than vacuuming there is also a procedure of heat ‘blowing’)
It could also possibly be a leakage path (caused perhaps by moisture in the air) or some conductive dust particle which has developed into a ‘habit’.(in which case you may be hearing the sizzle of charge leakage which is continuously being recharged by the copper charge ring) which is why there would be no problem for music playback or performance.
I would not want to speculate any further and I am not an engineer, but if you wish more feedback or discussion feel free to contact me.