My furnace has a vibration for about a second at the end of the heat cycle.
I agree with @nomind as far as removing the panel on the furnace and inspecting the the blower.

However, I am thinking the fan reaches a particular speed and the balance of it hits a resonant frequency as it slows back down through that speed.

I would suspect lint has built up on the fan and is affecting the balance of it. Remove the inspection panel and clean the fan thoroughly with a vacuum. If the vibration continues after that I would suspect a worn bearing but after only 6 years of use that would not speak very well of your brand of furnace.

Good luck!