I have the heat set at 68 degrees all day long, I never turn it down at night.

I have a INTERTHERM furnace, the model number on the internals is E2EB-020HB. The furnace seems to work fine during the day but early in the day usually 2-3 a.m. it will start running then make a annoying humming noise followed by a knocking sound. It almost sounds like something is hitting the side...