Hi Duncan and Alasdair
Yes agreed. This sounds like the bleed vent from the antisyphon as Alasdair has pictured. They shouldn’t really emit water but it doesn’t matter if they do. It the a symptom of the non return valve sticking. They are a cheap part to replace if you want to stop it: I replaced mine as I didn’t like the black streaks the water can leave down the hill.
The exhaust blocking advice is erroneous.
NA Tech Sec.