strange microphonic sound after amp is turned off

Hi all. I am a longtime Audiogoner but a tube amp newbie. I have had my first tube amp for only about six weeks now. Today I noticed something unusual that has not been happening previously. After turning the amplifier (Copland CTA506) off, after about 6 or 7 seconds (as the capacitors finish discharging?) there is a gentle rumbly/staticy sound out of the left speaker. Goes away after a few seconds. Not very loud. Doesn't seem to happen when I turn the amp back on, and music playback seems unaffected (at least so far).

As a musician the thing I liken this to is when you move an ungrounded microphone cable around on stage and you get some sound out of the amp.

I've checked all of the cable connections (power, interconnect, speaker) and they are all fine.

My only other components are an EAR Acute CD player (older model straight into the amp -- use the CD players volume control) and an Audience Adept Response AR-6T power conditioner.

Your advice is most appreciated.


Oh, and I forgot to mention that I have a dedicated line feeding the system and it is very well implemented so this is not an issue with an improperly grounded mains line. Thanks.
I don't think it is any thing to worry about. It might be the tube,try swapping the left and right channel tubes and see if it it changes to the other channel. My tube amps(Quicksilver)take a few seconds to shut off after power is off.
If your amp is sounding perfectly you don't have to worry about it. Amps have components which store energy, that energy takes a few seconds to bleed off.
Tubes can give off strange noises when new. If there are still low hours on the tubes, it might be one of the tubes "crackling." This happened to me for a short time and at random.
This is very helpful. It didn't seem like it was a big deal but it is good to get confirmation. I will monitor it and see whether it changes. Thanks all