Underfloor heating manifold hissing noise

[Greggy123]

Hi, I have part underfloor heating & part radiator system.
The boiler is losing pressure, so I feel there is a leak somewhere.
I've noticed the underfloor heating manifold hisses around the flow gauges when the heating is turned off (when cooling)
Is it normal to hear hissing at manifold or could the pressure drop be coming through air leaking. No leaking water is visible at all.

 

[Boilerman2]

NO IT ISN'T

It is likely that you have a leak on one of the UFH ccts, if the leak is not visible it is likely to be underfloor

Start by thinking if any work has been done in the UFH area that may have involved drilling into the floor or nailing of floorboards etc?

 

[Greggy123]

Can you eloborate?
Boiler has only just started dropping. No joins in underfloor circuits so I'm struggling to accept that it's leaking from one of the circuits.
Why would there be a hissing noise at manifold from a leak somewhere underfloor ?
I thought leak would more likely be on radiator circuits or air in underfloor heating system