new rigid fan duct is very loud

I have a bathroom fan that used to be vented by flexible aluminum ducting through the soffit. But I was getting ventilated soffits to work with an attic ridge vent and it seemed ill advised to dump all that humid air right under the soffit vents. So I switched to a roof vent.
When I ran the ducting for the roof vent I used rigid aluminum ducting instead of the flexible stuff, since I always read that the rigid ducting has a lower resistance to air flow. I used a 1 foot section of flexible ducting right at the fan exhaust port and then about 1 foot more of rigid ducting, and then an 80 degree angle bend up to the roof (about 2 ft) and then a 40 degree angle bend or so to connect to the roof cap.
The fan itself is a quiet fan. And with flexible aluminum ducting out through the soffit it was quiet. You basically couldn't hear the fan if you weren't in the bathroom. But with the rigid aluminum ducting it is much louder. What can I do to quiet the rigid duct? Or is the only quiet solution to rip out the rigid duct and use a flexible duct instead?
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snipped-for-privacy@cam.cornell.edu wrote:

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How did you mount the duct? Sounds (so to speak :) ) like it's acting as a drum. Try mounting it on isolated mounts--either hang it w/ something like plumbers tape or use a soft washer between the hard mount point and the duct to isolate it.
Wrapping it like a water heater blanket could help, too (as well as minimize condensation).
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Glue something heavy to it, like sheet-rubber.
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as we get older and fussier about sones, it's time to spend the $200 on the exhaust fan inline in the attic itself.
look at the fan options at
http://www.energyfederation.org/consumer/default.php/cPath/30_406
interesting other options for energy saving at http://www.efi.org /
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I listened to the fan some more last night and while it's definitely a lot louder than it used to be with the flexible duct vented out the soffit, I may be misleading when I describe it as "very loud". It's all relative. The noise sounds kind of like air flow in the duct. It's a kind of echo like sound, a sort of howling sound. The damper flap is not stuck.
Regarding an exhaust fan inline, I had such a fan in my kitchen. It was extremely loud. The new panasonic fan is rated 2 sones and was VASTLY quieter than the inline fan before I connected the ducting to the roof, and still definitely much quieter with the ducting hooked up. The ducting did make it a lot louder. If the ducting is a source of noise I don't understand how moving the fan to a different site will help as you still need ducting. Both the fan in my kitchen (which is not the topic of this post) and the fan in my bathroom (which is the topic of this post) are very quiet fans when there is no ductwork. Both of them got quite a bit louder when I attached rigid ductwork.
But the bath fan (which is rated at 0.8 sones) was very quiet when it was vented via flexible ducting out the soffit and is significantly louder now. The fan was the same. Getting an inline fan is clearly not necessary to make it quiet because the fan I have used to be quiet with a different duct configuration. Could the problem be the roof cap. There was an intermediate configuration where the fan was connected to the roof cap with a serpentine run of flexible duct that made a 90 degree turn right at the fan exhaust port (bad!). This was also louder than the old setup that ran to the soffit, but not as loud as the rigid duct.
I did some testing to assess whether the fan noise due to the rigid ducting had to do with the mounting or support of the rigid duct. I had my wife listen to the sound of the fan while I held parts of the ductwork to damp any vibrations. Nothing I did to the ductwork made any difference. So attaching a rubber sheet to the ducts will presumably have no effect. (It's got less mass than I do.) It almost seems like the only thing that could have an effect would be something that alters the air flow pattern.
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Is it possible that when you installed the new duct you accidentally caused the damper flap to not open? Try removing the fan motor and stick your hand up there to see if the flap is moving freely. If the air cannot blow out the duct the fan will make more noise than usual.
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