ABS operation

[BackintheFold]

My brakes are clearly playing up.
Everything is fine if I switch the ABS system off.
With it on, as soon as I brake (when moving) I get a weird valve-fluttering noise from the brake area (ABS unit and/or the" bomb")
It keeps making a noise like its trying to pressurise or a relay is fluttering until I switch the ABS off.

Any ideas? Is it likely to be the sensor gaps needing adjustment, or the sensor "ring" needing cleaning somehow.
Is it possible to check the sensors somehow? I don't have an oscilloscope so it would be by measuring voltage or resistance while spinning each wheel
Off the ground...
Thanks in advance for any/all pointers.