Recently we have had introduced to our Hydronic system - a noise - similar to a whistling noise. This noise is apparent all throughout the house - where the Hydronic panels are located. When the Hydronic system is activated about 5 minutes into its operation we have this whistling noise through the pipes and panels. Like a plane landing - where the noise diminishes as it goes on. After about 5 minutes - the sound disappears - quietly to nothing... I have bled each panel in the house to try and remove what maybe an airpocket somwhere - however this has made no difference... BUT what I did find tonight - was as soon as the Hydronic system started - I went out to the boiler and just lifted the pressure release valve - this made a squeaking noise thoughout the house and it released water through the copper pipe (attached to the release valve) and the noise was no longer there - much quicker than waiting for the five minutes - each time... So I feel I have the answer - which is located at the pressure release valve on the boiler (outside) however - what can I do to fix this...? Is there an auto release valve that should be working on the boiler and it is not? Anyone in the know - this would be greatly appreciated - and will bring peace to my mornings - instead of putting up with the noise..