C|24 Diagnose Problem of Low Thud Through Sub Output Channel 3 Only

[Willard]

I have a C|24, purchase late 2007. In the past month I’ve been getting a low pulse/thud (about 50bpm) through the sub. Here’s the details.

Channels 1 and 3 out of C|24 connected to KRK S10. Then feeding through from the KRK S10 to two KRK v6s.
I hear the low pulse/thud in the sub only.
I only hear the thud from channel three, not channel one.
Pro Tools (9) is not booted at the time.
When Pro Tools is booted, changing the volume does not change the volume of the low pulse/thud, it’s constant.

The C|24 seems to operate normally, nothing unexpected.
If I power down the C|24, I no longer hear the pulse/thud.

My guess is that it is the power supply, but would like to hear from anyone who has had similar issues or can suggest any trouble shooting I haven’t thought of.

Thanks