I am hearing loud knocking noises inside the air handler unit of my central air/heat unit located in my attic. This knocking is intermittent and I have only heard it while the unit is not running. I did not notice the noise until the winter and I started using the heat. Any ideas of what this could be? It is definitely in the unit itself and not the ducts. Thanks.

Yes, I am going to take the liberty and assume you have a heat pump system in your home. The heat pump uses a reversing valve to change the directional flow of refrigerant through your system. To change the indoor coil from a heating to a cooling coil there is a check valve in place to keep the refrigerant flowing in the direction chosen by the reversing valve in the outdoor unit. In the off cycle the reversing valve is deenergized the flow of refrigerant stops and pressures equilize (this flow of refrigerant will cause that check valve to chatter) As long as the chatter does not exist while the unit is running you have nothing to worry about,
P.s. let me know if you do not have a heat pump system as there is no explanation for that noise otherwise,.
I am a liscened air conditioning tech, and have 15+ yrs in the field, I hope my knowledge can be of help to you...