Weird Furnace Humming / Grinding Noise

[Maxx Mazda]

I’ll try to get a video, but here’s the scoop:

Home is a 2015 build. Central HVAC with a Lennox furnace and HRV system. Furnace will make a very weird humming/buzzing noise for 9 seconds, after heat is commanded, but not every single time. Once the noise stops, the blower motor kicks in and heat is felt through all the ducts. It’s a loud humming noise akin to what I imagine a giant kazoo sounds like. Very similar to the noise you get blowing across the top of an empty glass jug. Standing directly beside the furnace, the noise is barely noticeable. Farthest away, in an upstairs bedroom, it is loud enough reverberating in the air vents to wake the dead. Super strange.

Here is a sequence of what I observed standing at the furnace with the front cover off:

- Heat is commanded
- Burner blower motor kicks in for roughly 10 seconds
- Gas solenoid opens
- Burner lights off (THIS IS WHEN THE NOISE STARTS)
- Noise lasts for ~9-10 seconds then stops
- Furnace blower engages and blows heat to all the registers

Here’s what I’ve learned so far:

- Noise is intermittent
- Furnace works great, and does not show any error codes
- Not an electrical buzzing noise, the transformer is silent
- Happens intermittently, about 80% of the time
- Does not matter if HRV setup is disabled or not
- Does not matter if humidifier is engaged
- Not a mechanical “grinding” noise as far as I can tell, such as from a fan or faulty bearing etc
- Seems to coincide with the initial light off of the burner assembly

This is really driving me nuts guys. I’ve checked there is NOTHING loose or rattling around. The only thing I can think of, is it possible this noise is from the air moving around the burner itself? It is VERY similar to the buzz you get when you blow across the top of an empty water jug.

I’m scratching my head here... if anyone can offer any suggestions, I’m open to them. Once the front cover is off the furnace there isn’t a lot of access to any of the components without going in deeper. I am mechanically inclined and very handy with a wrench, so I’m able to delve in deeper if need be.

Thanks Beyond!

Edit: Managed to capture the noise on video through one of the louder upstairs ducts.

[Darell_n]

I’ve had my metal return air grills resonate exactly like you described, blowing across a beer bottle. It’s a pitch that is hard to locate by ear, but stick your head near them and push them by hand until you find the culprit.

Edit: re-read your problem, sounds like my garage heater when it lights up for the first 5-10 seconds. Some kind of turbulence through the heat exchanger.

[revelations]

change your filter and observe

[ExtraSlow]

Does the draft inducer run then? Seems to be the time it would.

[Maxx Mazda]

Managed to catch it on video. Original post updated. Filter was replaced. No change.

[firebane]

I would be looking at the fan blower motor. Sounds like a bad bearing or something going.

[prae]

Your shit is haunted- time to move. But yes as has been said, you should look into the inducer motor. As your video title suggests it's likely some kind of weird resonance that is amplifying the sound of a bearing on it's way out.

[revelations]

Try closing a vent or two while this happens in the nosiest area and see if it makes a difference.

"Standing directly beside the furnace, the noise is barely noticeable. Farthest away, in an upstairs bedroom, it is loud enough reverberating in the air vents to wake the dead. Super strange."

[redblack]

I’ve had a similar noise, turned out to be the inducer motor fan hitting the side of the housing. Shimmed one of the mounting points and the noise went away.

[legendboy]

sounds like a bearing at rpm, i would look at the inducer motor as mentioned above. can always install rubber washers on the inducer motor or main fan