How can I reduce loud knocking in my heat ducts?

I moved into a new home recently, and the knocking sounds made by the ductwork when the furnace runs are so loud that they wake me up. I wouldn't mind light knocking noises but this sounds like somebody is banging pots and pans right next to my bed. The knocking only happens while the air blows. There is one loud knock approximately every 10-20 seconds.

The funny thing is, the knocking only occurs at night. Right now it's day time, the furnace is blowing air, and there is no knocking. I believe it might have something to do with the fact that I keep the temp at 70 during the day but only 62 over night (I like it cold when I sleep), so the metal ductwork does more contracting and expanding during the night than during the day. Anyways my question is, how can I eliminate or at least reduce these loud knocks so I can get a good night's sleep?