155x continual running Water sound

I have just bought a flat that has a Powermax 155x boiler. When it's in operation There's a continual sound of running water which seems to be from a small diamond shaped see through "valve" on bottom left outside boiler with water running through it.
Why is it making so much noise and is this a major problem as I will need to sort this out with the seller of the property if they knew about the problem.
Regards
Bazza