My Martin Logan Center Speaker makes a high pitch sound (very low level) about 2-3 minutes after my system is turned off. It last for 5-10 seconds. I assume this is caused by the power being disspated as the 7500 amp I have turns off ( capacitor) discharge. This occurs when the Cinema i speaker is attached to any of the 5 output channels.

I do not hear this from any other speaker in the system (ML Vantages, NHT rear surrounds).

All speakers sound fine when operating.
Is this normal and what is causing it?

Joel