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I replaced the blower motor, blower wheel (squirrel cage), and run capacitor in my Goodman GMS80904BNBB furnace (1994 model I believe) because it showed signs of bearing and capacitor failure. First run with the new parts it blew air through the system at a wonderful speed; but it hummed, vibrated, and clanked loudly.

I was able to isolate the sound of the clanking to the control board having a loose mount, securing that made the clanking go away. Now I'm left with a vibrating/humming noise that I've isolated to be coming from the combustion chamber but only when the cooling is on and the blower is at full speed. The sound only comes from one side (the right-hand side when looking at the front of the furnace upright, if that makes a difference). The sound is about 75dB right next to the source of the humming (noise floor is about ~60dB for reference). The sound does not appear to be coming from the blower motor/wheel/housing, hopefully ruling out an imbalanced wheel/shaft/etc (the whole blower chamber actually runs very quietly and smoothly).

I have not tried it with the furnace heater on because it's 95 degrees outside, but I could if necessary. I do not remember this humming before I replaced the motor and wheel, the system has been operating quietly up until a few weeks ago when it started to grind, sputter, not start, etc because of the motor bearings. I believe this new motor produces more RPMs than my last motor (old motor model deprecated for the newer one) but I could check to make sure if that would help.

My question is, what could be humming and what would it take to fix it? Is there a component I'm unaware of that is operating in the combustion chamber when the air conditioning is on? Is it just harmonics? Is it really just an imbalance in the blower wheel that's being amplified at high speeds? Could I somehow reduce the blower speed to make it quieter?

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  • Can you wire your blower motor for different speeds? most blower motors have vibration dampers of mounts that cut down of vibrations. Did you check for them? – JACK Jun 26 '20 at 21:05
  • @JACK The new motor came with rubber grommets on the bracket arms and I remember them being installed correctly but I will double check this weekend. It is a 3 speed motor, so from what I've read I could wire it differently to use one of the lower speeds instead. When the system is between A/C cycles, the blower goes to a slower speed and the humming goes away. – Andrew Larsson Jun 26 '20 at 21:31
  • If the air flow can make a whistle like a piece of gras between your thumbs and blow on it when things are just right it makes a heck of a noise. – Ed Beal Jun 26 '20 at 21:46
  • do you have a seasonal damper? the one on my house can work itself loose and flap in the breeze, making a rapid clicking/scraping/tapping sound. Solution is to cut it hard on/off, or tape the level down in an intermediate position that prevents the flap edges from touching the duct walls. – dandavis Jun 27 '20 at 19:21
  • @JACK The rubber grommets are good and installed correctly. – Andrew Larsson Jul 1 '20 at 19:48

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