Furnace On/Off Banging noise
Sounds like the ducting is flexing when the blower kicks on. You're going to have to find which section of duct is moving, and either brace it (reduce movement) or insulate it (dampen the sound).

To locate the source, you could have a partner turn the blower on/off at the thermostat (obviously when the system is not calling for heat/cool). Then try and narrow down where the sound is coming from. Try holding your hand against surfaces of the duct (make sure it's not hot first), to see if you can feel the movement.

Unfortunately, there's not much more help I can provide, without actually being there.