My heat/ac unit makes a knocking sound repetitively when the unit is idle, then when the unit kicks on, the knocking fades away and runs like a champ. Then once the unit kicks back off the knocking starts to appear again; slowing knocking at first and then getting faster and faster until it reaches it's maximum knocking speed which is equivalent to the sound of a helicopter flying. Sometimes the noise is louder than others, but has the same patterned knocking sound.