Speakers.
Your analysis should have pointed you to the same conclusion. :)
I've had a similar problem with two different sets of Klipsch speakers
(will never buy those again!). Even with the system turned off (which
effectively eliminates the sound card, but to completely satisfy your
curiosity, unplug the speakers from the soundcard and observe that you
still have the problem) there would be static noises coming from the
speakers, but mainly from the sub-woofer. My sound would also diminish
when the static was occurring. (The "swelling" is just the sound level
returning to normal.)
this problem also occurs to me....
and culprit was the speakers.....
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basumarpo1610