Noise??? Hell, worry about the 20 year old ERAU grad flying the thing.
If a 1900 serves a route, it means just one thing: it's not too far to drive.

So, if radiation exposure on aircraft actually exists (which it does, unless you're the kind that believes a 757 never flew into the pentagon), how is it an urban legend?

You get more radiation on an aircraft than operating an x-ray machine. Of course it's a negligble amount. But I said that to give effect to the fact that losing hearing while being on the beechcraft is such a minor issue, IMO.

Which US carriers fly this aircraft?

It was fun, although slightly concerned by the extra 20 minutes flying it took to get back into the headwinds since route is getting out towards the range for the a/c.

One "big" operator is Gulfstream International, which operates as Continental Connection in Florida and the NE, as well as for other airlines.

Not US, but Air Canada has several feeder airlines that fly them down to the US, as well as domestically.

That is misleading. An Xray concentrates the radiation in a small area of the body. The radiation you receive while flying is spread out over the entire body so the amount of radiation received at any one point of the body is much less than what you get from an Xray.

Another one is Skyway Airlines (Midwest Express Connection).

Great Lakes, mostly out of Denver (don't ask how), is another large one with the B1900.

Colgan has them for EAS service under US Express in the east.