We recently had a RadonAway 501C fan installed for sub slab depressurization in our 1923 Cape. Our readings dropped immediately from 6 - 13 (average 9) to 0.3-1.5 (averaging 0.7)
We did expect that the fan would make noise - especially as it is a "high suction" at just under 4"wc. And indeed, the noise of the pipe in the basement is not unpleasant.
However we do get an additional rhythmic/reverberating woo-woo-wo-wooo. It's not obnoxious like the videos I've seen of 10 year old fans with toasted bearings, but it is super annoying - especially at night in our very densely developed neighborhood.
I liken the noise best to blowing over the opening of a bottle or jug.
It is very regular and does not appear to be tied to wind or weather, though it might be slightly more noticeable when it's raining.
It seems to originate outside as it is very faint in the basement, but very noticeable in the second floor bedrooms which are effectively at the same level as the exhaust as the rooms have slopped ceilings.
The installer came back with a cheap muffler (3" diameter pipe lined with 1/4 or 1/3 inch black foam) it made it slightly quieter but honestly not in any meaningful way.
What might be causing this reverberation"woo-woo" and how might I address it?