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Discussion Starter · #1 ·
This problem has just developed over the last couple of weeks. AC is otherwise working fine. When the AC cycles off, there is a thud that comes from inside the return vent. Have tried everything I can do to locate the source of the noise, but to no avail. This vent is located in our kitchen, just above where the air handling unit is located in the finished basement, below. The noise is not coming from the air handling unit. Have stood down there several times while the AC cycles off and I can faintly hear the noise coming from upstairs. I can only hear it upstairs near the return vent.

It sounds like metal in the duct work flexing when the AC cycles off. I did a little fiddling around and it only happens when the air filter is installed. Air filter out, no noise.

Last night, I taped my phone (upside down) at the top of the duct just inside the return vent. I turned video recording on, put the filter in and closed the grate to see if I could tell where it was flexing when it cycled off (side note: i also tied some twine around the phone case in case the phone fell, so I could retrieve it). As it would go, the thud didn't happen either time I tried it with my phone taped to the top of the duct (got some nice video though!).

This led me to think the metal was flexing there and the weight of the phone taped to it kept it from happening. I don't know. The spot where I had the phone didn't flex when I pressed against it with my hand, but it did about 6 inches away from that spot.

Is this an issue anyone else has encountered? At this point, it's just a nuisance, as it doesn't seem to be coming from any mechanical part of the AC and the AC is still working fine. I don't know how you'd go about fixing it other than ripping out the drywall and installing new duct work. Not gonna do that.

Here is a video I took when the AC is cycling off. It sounds louder in the video than it does normally. (happens at about 12 seconds)

 

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Discussion Starter · #2 ·
Well, I just did some Google searching (which I should have done first) and I guess this not that uncommon? I'm 43 and have never had this issue in any home before, so it was baffling to me.

Guess there's not really a way to fix it.
 

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It's called "oil canning" which means a noise similar to what the old fashioned bottom press oil cans made when you pressed your thumb on them to squirt out the oil. It's one drawback with having register filters as opposed to at the blower inlet.

If it's a filter behind a register that allows access to the inside of the duct, you may be able to cut a piece of wood or conduit to an exact size that will wedge tightly inside the duct to stop it. It's an easier fix from the outside but you likely don't have access.
 

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Discussion Starter · #4 ·
It's called "oil canning" which means a noise similar to what the old fashioned bottom press oil cans made when you pressed your thumb on them to squirt out the oil. It's one drawback with having register filters as opposed to at the blower inlet.

If it's a filter behind a register that allows access to the inside of the duct, you may be able to cut a piece of wood or conduit to an exact size that will wedge tightly inside the duct to stop it. It's an easier fix from the outside but you likely don't have access.
Thanks! I do have some access reaching in through the hole that's covered by the filter. I just have no idea where the metal is flexing, and I can only reach so far down. House was built in 2015 and we moved in last summer and it just started the last couple of weeks. Beats me.

Guess we're just stuck with it!
 

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If you're using a MERV8 rated filter or above, try a lower rated ... start with a plain vanilla bottom of the line fiberglass one to see the possibility.

You may be able to turn this lemon into a blessing if you can come up with the correct filter that prevents the oil canning until it gets slightly clogged and needs to be replaced. Nice thought, what?
 

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Discussion Starter · #6 ·
If you're using a MERV8 rated filter or above, try a lower rated ... start with a plain vanilla bottom of the line fiberglass one to see the possibility.

You may be able to turn this lemon into a blessing if you can come up with the correct filter that prevents the oil canning until it gets slightly clogged and needs to be replaced. Nice thought, what?
Got a 3M filtrete. Package says 800 microparticle reduction level, which is on their lower end. Can try an el cheapo one see if that makes a difference.

Thanks for the tip.

It does not make the noise when I take the filter out.
 

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That's a MERV9 rated filter you have. I have hope that you can experiment your way out of this problem by going going back to a basic filter and then perhaps over time try a Filtrete300 (MERV5) and 600 (MERV8) if you think you need to.
 

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Discussion Starter · #8 ·
That's a MERV9 rated filter you have. I have hope that you can experiment your way out of this problem by going going back to a basic filter and then perhaps over time try a Filtrete300 (MERV5) and 600 (MERV8) if you think you need to.
Great, thanks! I'll grab a different filter in the next couple of days and see how that does. Thanks for the tip!
 

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Discussion Starter · #9 ·
That's a MERV9 rated filter you have. I have hope that you can experiment your way out of this problem by going going back to a basic filter and then perhaps over time try a Filtrete300 (MERV5) and 600 (MERV8) if you think you need to.
Thanks again. This solved it. I'm down to the most basic filter you can get, but it hasn't made that noise in a while now.
 

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With that return register and duct being that close to the unit, by installing a filter with a lesser MERV rating other returns in the system ( if applicable ) will likely have less air flow and any on the far ends of the return system may be reduced to near zero air flow. If the industry sizes return ducts as they do supply ducts, distant smaller duct returns may have been getting very little flow with the original higher MERV filter.




Edit: Edit:
 

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Discussion Starter · #11 ·
With that return register and duct being that close to the unit, by installing a filter with a lesser MERV rating other returns in the system ( if applicable ) will likely have less air flow and any on the far ends of the return system may be reduced to near zero air flow. If the industry sizes return ducts as they do supply ducts, distant smaller duct returns may have been getting very little flow with the original higher MERV filter.




Edit: Edit:
ummm, yeah, don't understand any of that :)
 
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