Tankless water heater - Noisy?

Does anybody here know just how much noise a tankless water heater puts out? I know the fans on these things make some noise, but since this unit will be facing the good folks next door, I'm a bit worried that whenever I take a shower they'll think a plane is landing on the roof.
I'm considering a Takagi, but some of the others look pretty good. Unfortunately, none of the manufacturers put noise ratings on their sites.
I'm also planning on installing the water heater in place of my old one, which is outside, and enclosed in one of those metal sheds made for the purpose. I'm wondering if there will be any problems, such as code violations.
Thanks for any input.
Pagan
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Stiebel Eltron claims that theirs are virtually noiseless. If you call them they say that they are around or less than 6 decibels. You can get them at online retailers like this.
http://www.plumbersurplus.com/ProductList.aspx?ID 0
I don't know about the code???
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