This is gonna get extra super nerdy and long, so buckle in.
A couple years ago we had a heat pump installed. Our gas furnace was 23 years old and working fine, but there was an unusually tall stack of incentives and rebates for getting a heat pump, and that would also give us A/C, and it's less traumatic to replace a furnace on our schedule rather than the furnace's.
I had the installers cut into the air return duct in the basement where it passed by the wall, to provide an air return inlet down there rather than just the one on the main floor. That worked well, but the plastic grille they installed doesn't take kindly to the trick of pushing a twist of paper napkin between the slats with a few drops of essential oil on—sandalwood or rosemary or whatever—to help out after a smelly cooking operation or otherwise like that. The plastic goes sticky in re the oil, and doesn't let go the remains of the napkin.
So I thought I'd replace it by a metal one, what I picked up at Home Despot. Made in Canada; I was shocked (tho I probably shouldn't've been; registers and grates and grilles and suchlike are still an industry here).
Its screw holes were a fraction of an inch closer together than the plastic grate's ones. Not enough to stop me fitting it, just one of the screws went into the wall hole slightly cockeyed.
The next morning I was putting breakfast together when I became aware of a noise, sort of a droning/ringing. Maybe a hydraulic pump, one of the nearby restaurants getting its grease traps cleaned or something? No such trucks out in the laneway, and sticking my head outside made it clear the noise wasn't out there.
It got louder when I took a basket of laundry downstairs to the basement, and I was instantly time-transported nearly 40 years back to…
…I grew up in a U.S. suburb. A decade and half before we came along, our house had been built to order for a General Electric macher. Everything in it came from GE that could've; appliances, switches and outlets, and the two furnaces—a vertical in the basement for that and the main floor; a horizontal in the attic amidst the itchy rock wool for the upper floor—each with a de luxe GE electronic air cleaner (they didn't work by the time we lived there) and corresponding dual A/C (which did).
Even the thermostats were GE items, and the install had been posh as well. Across the upstairs hall from my bedroom were two full-width air returns, one near the floor and one near the ceiling, presumably to optimize temperature control in both heating and aircon seasons.
A lot of this was wankery; the house wasn't small, but neither was it anywhere near big enough to warrant the fancy GE intercom with a speaker in each room and one next to the front door. And the climate wasn't extreme; I reckon the house would've been fine with one each furnace and A/C, appropriately specified.
Nevertheless, there were two. By and by, the upstairs furnace needed replacement at a few over 20 years old, as they do. Dad's father was a very good HVAC engineer, and he helped guide the selection of the new furnace and equipment.
The furnaces of the late 1980s were rather more efficient than what had gone before. One of the differences was higher-speed, direct-drive blowers to move more air. Very fine.
Only once the new furnace had been installed, there was a weird new noise when the blower was running, but only sometimes: a sort of ringing/droning, always at one pitch, but varying in volume seemingly at random. When it happened, it heterodyned.
The installers came back a few times trying to find and silence it, without success. The furnace and its install were checked thoroughly, multiple times. Grandpa suggested soft rubber isolators between the furnace and the attic floor, and flex joints in the supply and return ducts. Made perfect sense, so these were added, and the noise carried right on.
Eventually we just gave up and lived with it. It was annoying, but the furnace worked fine and we, the installers, and even grandpa had run out of ideas about the noise.
Back to 2025: I quickly traced the noise to the metal grate I'd installed the night before, which went quiet as soon as I put a hand firmly on it.
It was the same noise, minus the heterodyning because 2025-me was dealing with only one grate. It was those two air return grates fluttering at high frequency against the wall in sympathy with the vibrations from the high-speed blower, making alllllllmost the same note, hence the heterodyning. Putting strips of rubber or Velcro or suchlike between the grates and the wall would've silenced it, sure.
I removed the metal grate, took it back to Home Despot, and put back the plastic one for now.
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