For the last 2 months when my neighbour has her central heating system on (ALL THE TIME) !! I can hear a series of loud knocks coming from what appears to be inside the wall close to where her boiler is situated which is in her kitchen. The knocking starts almost as soon as her heating comes on, as the boiler heats up the knocking gets louder and louder then when the boiler goes off the knocking gets quieter until it stops altogether. The only problem is her boiler kicks in every 3 minutes so the knocking is almost constant. It is driving me absolutely mad as it goes on all day long and actually sounds like the noise is in my house (it's not my system as I hardly ever put my heating on). I have spoken to her about it and she has refused to admit that there is a problem, she has had an engineer in who I believe has bled the rads and released any trapped air & told her the system is working just fine, he also put some 'gunk' in the tank to clean the pipes. The extraction vent is situated on the wall just outside to where her boiler sits in the kitchen and from the outside it sounds like the knocking is coming from this vent, the system has been fitted for 10yrs but the knocking only started 2 months ago, as far as Im aware no pipes have been touched recently... anyone got any ideas as it's driving me mad now ?