Just recently moved into a new house.

The furnace is a older model (mid 90's) but seems to work OK.

One problem I notice is that when the system kicks on usually after about 1-2 minutes there is loud bang that comes from the bottom. This bang also happens after the system kicks off as well.

See the video below to show what I mean.

Is there anything I can do to mitigate this? I've tried bracing certain parts of the metal, but it doesn't really seem to help.

Any thoughts? Thanks!

https://youtu.be/T0YSbpQalbI (bang @ 25 seconds)


Sounds like the ducting is flexing when the blower kicks on. You're going to have to find which section of duct is moving, and either brace it (reduce movement) or insulate it (dampen the sound).

To locate the source, you could have a partner turn the blower on/off at the thermostat (obviously when the system is not calling for heat/cool). Then try and narrow down where the sound is coming from. Try holding your hand against surfaces of the duct (make sure it's not hot first), to see if you can feel the movement.

Unfortunately, there's not much more help I can provide, without actually being there.

  • sounds like the duct installation company may have undersized the duct work. If you can find the duct in question making the noise you can screw a brace to the duct work. Myself, I would at that point have an HVAC co.do the work. Tell them the problem and they can fix it.
    – d.george
    Mar 20 '17 at 9:31

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