Anyone know what this means? This hissing sound isnt constant, it is
irregular: It isnt related to the music or any movement of the cone
or any signal put through it, though it doesnt happen when no sound is
put through the speaker.

Frankly it sounds more mechancial than electronic (as if there is
something behind there!) but as I say, it isnt related to whatever is
coming through the speaker though it does increase and decrease with
adjustment of the volume control. Andit is louder than most signals
put through the woofer itself??

Is it indicative of some pot in the crossover needing cleaning or is
there something else going on please? (I have seen reference to people
improving the sound of AR11s by bypassing the tone controls in the
speaker completely). This speaker is quite elderly and I would
imagine that if it does use mechanical pots, they must be fairly dirty
by now