Hissing Martin Logan SL3's

The electrostatic panels of my SL3s start making a hissing sound (white noise) after being used for a short while. The noise is not audible while listening to music at normal levels, but is audble between tracks and at low levels. The noise continues for about 10 minutes after the power amp is switched off, then stops. In order to try and isolate the problem, I disconnected the power amp from the speakers after turning it off (I thought maybe the tubes were getting noisy). The hissing did not stop. Anyone have any ideas about the problem ? The speakers are just under 3 years old. No smart comments, please, about looking for snakes !!!