What can I hear through my walls at night?

My neighbours recently had a boiler fitted onto the party wall of our bedroom. They have since had it moved into their bathroom, but I can still hear like a water/hissing sound when the bedroom is quiet i.e all night. Sometimes it is louder than others but I cannot pinpoint the noise through the wall. It is a coincidence that this noise was not apparent before the boiler was fitted. It is driving me insane!! They have been known to have the heating on all night and wonder if this is what I can hear.