Hissing when tap is turned off

[suz]

Hi,

I've just had a thermostatic concealed bath/shower valve installed in my bathroom. When I turn off the tap that controls the water to the bath, I can hear a slight hissing noise from the tap, just during the final part of turning the tap off.

I've tried adjusting the pressure by turning the stopcock & I've tried turning on all the taps in the flat at the same time (in case it is trapped air?), but no change.

Any ideas what this could be? The noise doesn't bother me but obviously if it could be a leak then this does bother me!

Thanks!