Diagnosis
That's almost always the booster pump running non-stop because it can't build enough pressure to hit its target. The usual suspect is a clog or restriction somewhere after the pump.
How to Fix It
The most common fix is a clogged post-filter (the last carbon stage). Turn the system off, remove that final filter, and try running it briefly without that cartridge installed. If the noise stops, you've found your culprit—replace that filter.
If That Didn't Work
Check the flow restrictor in the drain line (the grey tube). If it's kinked or blocked, the wastewater can't exit properly, which backs up pressure and makes the pump struggle. Ensure that line has a clear, downward path to your drain.
Heads Up
Letting it hum like that for extended periods will burn out the pump motor. If the quick fixes don't work, shut it down until you can diagnose further.