With that in mind I plugged my 'scope into the CLK and low and behold I have small, offset by 0.166v triangular wave of about 4.4kHz peak to peak 0.1v. It doesn't change with sampling frequency.
It's probably a clock input. The small signal you're seeing could be just the CLK picking up other signals on the PCB.
Hmm. What could I do with that?
Scroll down on this link to "External Clocks".