It does sound like noise to me.
Did it suddenly stop working, or has it always been a problem?
If so, what else may have changed in your home?
New electronics, and even failing (or brand-new) CFL bulbs can introduce noise into your system.
However, if the pump ALWAYS turns on, but only turns off half the time, I would look at the pump first.
As a simple test, unplug the pump, and plug in a simple (incandescent) lamp. Run your signal tests, and see if it always works. If so, try plugging the pump in through a filter.
Are you able to correlate the working/not working to specific times of the day? Perhaps a light or other device that is on only at night is causing the interference.
