Honey, there are gremlins in the house!

Like the saying goes, "Flying is the Second Greatest Thrill Known To Man, the First is Landing".

The most thrilling part of getting into a new house is the first night. Or any night, for that matter, until the dust settles. It's the time when you find out with excitement that the people that sold you the house forgot to mention that the neighbor behind the southern fence likes to play his electric guitar off key at 5AM, or that the guys across the street get their truck that is older than me out of the garage and start fiddling with the engine and revving it at about 11PM... You get the picture.

Then, deeper into the night, you start hearing noises. The one I'm talking about drove me nuts for several months until I figured out what it was. It's quite rare (so far), because the only HVAC units affected by it are the ones that are slowly ramping up and down.

I'm taking about loud bangs in more or less rapid succession, seemingly all across the house. Gremlins.

Hindsight is always 20/20, and now that all the facts are laid out before you it is quite simple to see what it is: it's the flat tin panels that comprise the ducts flipping back and forth when the pressure changes, all of them at different times. What makes it fun is that the sound pattern changes as the building ages - panels that used to be well insulated and thus somehow dampened are not anymore because of the insulation breakdown, and the more time passes, the more pronounced the phenomenon becomes.

It is also an indication of the fact that the pressure in the ducts is quite high, probably way above what it should be.

The best part? There's probably nothing you can do without significant amount of money or elbow grease injections, so might as well get used to it.

Proper fix, though, assuming the pressure is within reason, is to apply sound dampening materials to the outside surface of the ducts. I guess the trick would be to find a material that sticks to the surface and doesn't let it bang, and at the same time won't break down for a long time.

There are also products called "acoustic duct liners", but knowing how widespread the problem of undersized ducts is, I would be extremely cautious stuffing anything alien into the ducts, unless you're absolutely sure the duct itself is grossly oversized.

Which returns us all the way back to the heat loss and gain (a.k.a. heat load) calculation, but that's a different story for a different time...