When my furnace turns off there is a loud bang. It looks and sounds like there is air being sucked in through the ducts and then 'let go' all of a sudden. Any suggestions on what this could be and how to fix it?

It is a sheet metal sound you probably hear.
Is your furnace filter clear?
Are all your registers open?

You can call a professional and have the cross brake the offending piece of sheet metal.

If is is a sheet metal sound, this is called "oil canning".
Years ago (and still today) there are oil cans made that dispense oil by pressing on the bottom of the can and it makes a kind of "pink-ponk" sound because the bottom is convex and is designed to flex.
A large piece of sheet metal duct will do the same as it heats up and cools, making it expand and contract, or has a large amount of back pressure that makes it bow.
If it is the return duct or plemunm, it will bow in and pop as well.