My home is 6 years old and about 2 years ago I started to notice a knocking noise inside the wall while using the central heat. I have a 2-story home and the master bedroom is upstairs on the same side of the house as the heat pumps out side. I have two units (one for downstairs & one for the upper level). I only notice this while trying to sleep because the head of my bed is close to the outside wall that is shared with the heat pumps. Lately, I have had to turn off the down stairs unit at night in order to get some sleep. So I have narrowed it down to one unit. Again, this only happens in the winter while running the heat. It is intermittent slight but noticeable knocks. Any ideas what could be casing this and how to fix it?