# Thumping noise when you turn on the fan



## hvac122 (Dec 10, 2007)

Do what a tech would do and go listen to it and find out exactly where the noise is comming from. I can't seem to hear it from here.


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## Yoyizit (Jul 11, 2008)

dacoopsssss said:


> a loud thump thump thump.


Might just be a loose set screw in the squirrel cage. See if you can wiggle the cage relative to the motor shaft. 

Could also be a balancing clip has fallen off of the cage.

Could also be a defective motor shock mount.

What you can afford depends on how useful a damaged A/C is to you and your tolerance for this noise. 
I.e., pay me now or pay me later.


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## SKIP4661 (Dec 3, 2008)

May also be debris on the blower wheel, such as a plugged air filter etc.. I would check this first.


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## sceggel (Aug 29, 2009)

*blower wheel*

I have a trane hvac system and I had the same problem, only mine was more a vibration when the fan ran. It turned out to be connection on the blower wheel had come loose, so it was spinning out of balance. The solution was to replace the blower wheel. (I replaced the motor as well because it probably ruined the bearings in the motor.)


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## hvaclover (Oct 2, 2008)

thump-thump-thump

just like an out of balance washing machine.

First thing you should do is get a very bright drop light and hold next to the blower motor.

Look into the blower wheel as you turn it and look for a piece of paper or some thing similar stuck in the blower wheel.
If you can pull the blower assembly out completely you stand a better chance of seeing if something is stuck in the blower.

If there is nothing in the blower wheel than it's either the wheel or the blower motor.

Least likely is one of the mounting legs came lose.


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## diy_homeowner (Sep 8, 2009)

My HVAC was making a thump when the blower/fan turned off. It turned out to be a dirty air filter. Of course, one should never let their filter get that dirty before replacing :wink:

Sometimes it is the simple things ....


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## hvaclover (Oct 2, 2008)

I say you're mistaken. Just a dirty filter does not cause this. If you left the wrapper on the filter, that could get pulled in to the motor and cause the noise.

Or if you let the filter get too filthy it could get sucked into the blower, but it would have to be so filled with dirt it would look like a piece of carpet. 

I have been on several calls of this nature. 
I get a few new customers each year who swear they have changed the filter regularly.

When I pull out the filter to check it, it's encrusted in dirt. Customer gets a little embarrassed.

But in none of these scenarios did it make the motor go thump-thump-thump just by being dirty.


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## diy_homeowner (Sep 8, 2009)

My situation was without a doubt caused by a very dirty filter. It was only single 'thump' that was heard with the blower cut off, the filter was being pulled so tight against the blower it must have been creating a suction that was being released when it cycled off. It almost sounded like someone kicked the outside of the air handler down in the basement ... we could hear it while upstairs in the room above.

This is a bit off topic I guess since dacoopsssss reported multiple thumps.


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## hvaclover (Oct 2, 2008)

That was your return duct work flexing, not your filter.:whistling2:


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## diy_homeowner (Sep 8, 2009)

I never said the filter was flexing or that it was the source of the noise, just that it was the cause ... which it was.


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## hvaclover (Oct 2, 2008)

diy_homeowner said:


> I never said the filter was flexing or that it was the source of the noise, just that it was the cause ... which it was.


Then you are off topic. Please don't hijack the thread,


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## Jimmy s (Nov 22, 2021)

My blower thumps/pops one time when it starts, runs and blows fine. Checked it, nothing appears loose.


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