My house was built in 1955 and we just had our furnace replaced last January. A few weeks ago we heard a knocking noise coming from the wall in our bedroom. My husband went down to the basement to see if he could find out what it was but no luck. He thought it had to do with the sprinkler system (which is old and we've never used it in the 2 years that we've lived there) that is in the wall of the basement bathroom but we don't hear anything coming from there. He thought the banging noise could be pressure built up in the water pipes too, but we ran water from every sink and the noise came back.
We haven't heard it since that first night, but last night it returned and didn't go away! All night long the banging would start and stop! My husband can sleep through anything, I'm not that lucky! I went down to the basement to do some searching myself and I can hear it in the storage room and bathroom but it's very faint, not like the loud banging we hear in the bedroom. My thoughts are it's coming from the vents but we've been having an unusual warm fall and we haven't had the AC or the furnace on in over a month. In fact we haven't turned the furnace on at all this year. I am desperate and tired from lack of sleep! Do you have any idea on what this could be?
Our house is a 1 story with a finished basement, bathroom on main floor is to the sandwiched between the 2 bedrooms on the main floor at the back of the house. The noise is louder on the outside walls of the bedroom, not the ones shared by the bathroom or family room. The bathroom downstairs is directly below the master bedroom and this has a water meter looking device and the pipes and turn on switch for the sprinkler system, but the noise doesn't seem to be coming from there. It seems to be from the vents??

You might turn off the power to the inside unit and check the blower housing,wheel,and motor mounts to be sure all is well in that area.