How to silence your kitchen hood vent flapper:
As I stood in our new kitchen on a cold wintery night, I heard the outdoor vent flap for our new hood rattling open and closed. My wife asked “what’s that banging open and closed? “Thinking of the heat that I was loosing bothered me, but it was the recurrent annoying “thunk”….”thunk”….”thunk” that drove me to solve the problem. My solution has worked like a charm.!

Here’s how to silence your flap, flap, flapping flapper:
A flat head bolt, a threaded Hux nut, a magnet and some glue are all that’s needed.
1. The magnet is glued to the bottom of the inside of the flapper door in the duct.
2. The bolt is threaded into the elongated hex nut, and this is glued to the duct sidewall.
3. Wait for these to dry
4. The bolt is adjusted with a phillips screwdriver to be close, but not quite touching the magnet.

Operation:
When the vent hood fan turns on, the air pressure rushing out forces the vent to open, breaking the magnetic force holding the flapper closed. When the vent hood fan is shutoff, and the airflow diminishes and the flapper begins to close. As soon as the magnet nears the adjustable screw.. you’ll hear a single, authoritative solid “thunk”. The vent is held closed by the magnet and no more flap, flap, flapping.