follow up to ABS pump question W126 300SE (1989) UK

Following up on my 'oh my God its the ABS' pump question.
I was pointed in a different direction

pump and nothing else ie: the heater control valve opening and closing to maintain temp in car (try turning heater control dials to max hot and see if noise stops cycling) also is the noise present with ignition on or engine running? there is also an air pump behind the glove box which starts to run when you put the ignition on.

Turns out this is correct. Its this pump on the heater, that is punctuated by the heater control valve opening and shutting. If I modify the settings on the temperature wheels, I can cause the valve to stop operating. The pump continues to run tho. This all seems to be in order, tho I don't understand why the valve needs to keep opening and shutting. I can definitely hear it ticking away every several seconds when it is in this mode. Is that correct?
Cheers... Rob.
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Rob. Smith wrote:

correct amount of hot coolant to pass by to the heater matrix to keep the heater temperature at the selected level.
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