Non stop pumping noise when steaming ?

Our Russsell Hobbs Steamglide iron model 17880 carries on with
pumping noise when steam button is pressed ,should this be
happening ?
I'm sure that the pumping sound used to be intermittent when
steaming, am Iimagining this ?
What exactly should be happening when the steam button is
pressed , is there a fault if so how can it be corrected ?

Thanks

Winifred Birkett