I can hear this also from my Polk mains, very, very faint though. I assume that it is just normal capacitor discharge. I think that it actually sounds pretty cool, but I think that it is normal.
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Fronts: Paradigm Studio 100 v.4
Surrounds: Polk TC615i
Center: Paradigm CC-590
Sub: SVS PB-12Plus/2
Amplifier: Outlaw 7500
Receiver: Integra DTC 9.8
DVD/CD: Sony DVP-CX995V
HDTV: Sony KP-51HW40
RC: Philips TSU-6000