Very noisy water heater exhaust

I have a Bradford White TTW1 ("through the wall") water heater. It has a forced exhaust fan and is vented via 2" PVC to the adjacent wall - a total of about 5 feet of PVC. There are 3 90's and 2 45's on the way out. (Within spec according to the manual.)
The problem is that it is vented to my deck about 6 feet off the ground and is really loud when running. It's not just the air moving, but also has a definite resonant frequency.
How can I make this a *lot* quieter?
1. Replace with 3" or 4" PVC? 2. Some sort of muffler in-line or at the exit point?
Holding on to the pipe doesn't seem to affect the sound so I don't think wrapping or bracing it will help. The fan is single speed, or I would turn it down.
Anyone ever solved this problem before?
Dan
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Dan wrote:

I have one with 3" pipe and similar bends. It's noisy like a banshee too!
The noise is probably generated right in the blower blades and not much you can do about it. Muffling the exhaust may create a restriction which will prevent it from operating. Can you duct it upward (maybe 10 feet high)?
I solved the problem (for my needs) by putting a 24 Hr timer on the heater. It simply doesn't run during periods when the noise is disturbing.
Jim
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Dan wrote:

I don't think you can solve it. Thank God I have the inefficient pilot light type so it doesn't have the forced exhaust fan. But my furnace does, and that damn thing makes way more noise than the blower and near the end of the cycle it makes more noise than the burner.
If the problem is the noise out on the deck, you might be able to build a muffler and enclose the pipe until it was well above the deck. If you find a solution, let us know.
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