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Old 08-29-2021, 12:56 PM   #1
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Roof A/C Overhead Duct Rumble

Hello--anyone experienced this? 2007 CC 45' Intrigue, 4 slides. AC's have not run for a month and today when exercising the roof AC's I heard a previously unnoticed "air rumble" in the overhead air duct(s). To test, extended all 4 slides and turned on the bedroom #3 AC unit. Rumble was noticed primarily in that area. Turned off #1 and turned on units #2 (mid) and #1 (front) one at a time. Rumble primarily coming from running AC unit area but not coming directly from the vents-- more like from the ceiling area around the running AC unit. If all three AC's are running, rumble most strong from below all three units but you can also hear it from outside the coach. Also tried with only fans running (no compressor) with same result: mid frequency fluctuating light rumble that sounds like air is moving around an obstruction, but flow out of the vents seems normal and air is as cool as it should be with compressor running. I can't image how a partial blockage could occur in that overhead duct, which I assume is a common duct for all three roof AC units.

Any ideas would be most appreciated. Thanks!
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Old 08-29-2021, 01:12 PM   #2
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I'm not sure that I'd call the issue in my Bounder a "rumble" but there was significantly more noise and vibration coming from the rear unit until I girded up my loins enough to climb up onto the roof to investigate and found that the mounting screws on one corner had worked loose enough to allow the whole thing to shake and shimmy, no doubt the problem started small and worked the mountings loose progressively. My guess is that if I didn't replace the screws (and re-caulk the area) I would have eventually added an A/C unit to the occasional detritus one finds along the interstates of America.

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Old 08-29-2021, 01:30 PM   #3
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Originally Posted by Frankmerrill View Post
I'm not sure that I'd call the issue in my Bounder a "rumble" but there was significantly more noise and vibration coming from the rear unit until I girded up my loins enough to climb up onto the roof to investigate and found that the mounting screws on one corner had worked loose enough to allow the whole thing to shake and shimmy, no doubt the problem started small and worked the mountings loose progressively. My guess is that if I didn't replace the screws (and re-caulk the area) I would have eventually added an A/C unit to the occasional detritus one finds along the interstates of America.

Best Regards,
Frank
Thanks Frank. This is more of an air flow sound than a vibration and it is happening when any of the three units are running. My only thought is that maybe mice somehow got in there and built a nest but that vent is (or should be) air tight.You would need to get a camera and light on a flexible cable to check it out. Kind of like an air duct colonoscopy.
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Old 08-29-2021, 01:32 PM   #4
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I suspect you have debris in the blower wheels. Mouse nest maybe? Might be nothing more than dust buildup on the wheel.
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Old 08-29-2021, 01:36 PM   #5
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Thanks--good thought. But the #1 AC is new and it is as noisy as the others. It will just take some getting at the duct areas, which in this coach is a total pain. Without removing the AC unit, I have no idea how you could access the fan blower wheels.
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Old 08-30-2021, 06:03 AM   #6
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Thanks--good thought. But the #1 AC is new and it is as noisy as the others. It will just take some getting at the duct areas, which in this coach is a total pain. Without removing the AC unit, I have no idea how you could access the fan blower wheels.
Check the wheel anyway. It might be bent slightly. Runout can have the same effect.
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Old 08-30-2021, 11:04 AM   #7
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The ducting is an echo chamber and allows all AC units to pump air to all locations in the rig unless blocked at the ceiling output.

You can remove the cover of each AC unit and see some of the blower wheel. Just 4 phillips head screws each.

Perhaps, with all three running, you could determine which one might making the noise by being on the roof.
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Old 08-30-2021, 12:34 PM   #8
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Thanks--the strange thing is that this underlying noise is present no matter which one of the three AC units is running (or louder with all three running) which means that if the noise is coming from an AC unit, they all have the same issue that started at the same time. Not very likely, I think. The only common thing to all three is the ducting but the general position of the noise is related to the position of the AC unit running, but seems to be coming more from the ceiling area, and not out of the vents themselves. This is a good mystery.
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Old 08-30-2021, 12:52 PM   #9
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Just another guess at what it might be. I have seen air ducts that had a foil backing that over the years came off and whipped inside the air duct making all kinds of noise. If this is the case you will probably find some pieces of foil in the air discharge vent as times goes on.
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Old 08-30-2021, 05:21 PM   #10
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That makes sense. Now--how do you fix it?
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Old 08-30-2021, 05:52 PM   #11
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That makes sense. Now--how do you fix it?

That's the question,don't know if it can be without ripping the duct out and repairing. Or over time it will self destruct and blow out, not going to hurt anything as it does.

Not the answer you wanted to hear, sorry.
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Old 08-30-2021, 06:53 PM   #12
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From another AC Thread

I do not know if your noise is a wubba wubba thing.

One thing I learned tightening the bolts again - it's possible to twist the AC chassis and induce a "wubba wubba" noise that comes from the compressor making contact with the bolts that go through its vibration isolators. You can make the noise come and go by tightening and loosening bolts. You can also induce the cold side fan rub and make it go away. Tightening the bolts with the AC running is the way to go.

The above is from a thread in Systems Forum. https://www.irv2.com/forums/f54/some...ts-551970.html
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Old 08-30-2021, 10:38 PM   #13
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Thanks for this. The noise also occurs same intensity with only the AC fan running--compressor off. It really sounds like an airflow disruption. Have to talk to a CC tech in Oregon next time I am there.
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Old 09-05-2021, 03:59 PM   #14
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One last note. Today I drive the coach for about 25 miles and upon return started the forward AC unit. There was no "extra" rumbling as previously described. I have no idea what could have occurred to make any difference. Thanks to all who responded. Stay safe.
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