Panasonic's site used to have a "flow calculator" where you entered the CFM you desired, and then entered the number of bends in the duct and the number of feet of straight duct in the entire run.

It'd then spit out the CFM of the fan you needed to install in order to get the CFM that you actually want in the bathroom.

Are you getting a metallic tinny type of noise in the bathroom from the roof vent? You might be able to deaden it by applying a sticky membrane (or several layers) to the underside of the vent's cap to add mass and knock down the acoustics of the cap. Something along the lines of a Grace peel and stick membrane. Really just a recommendation. I've never had to deal with an issue like that before.

I thought I did use the flow calculator before ordering the fan Panasonic FV-15VQ5 WhisperCeiling 150 CFM last November. Our MB is 100 about s.f. in an L-shape with an 8 ft. ceiling. Just didn't want to have to run it for an hour to get rid of dampness. Funny, in the winter, it feels like a gale force wind when I step out of the shower, so it is definitely doing a job.

Yes, the rain noise sounds as if rain was hitting metal, though I think our contractor used a plastic vent which runs into standard round ducting in the attic. So the solution will be something to act as a sound dampener placed as near to the source as possible? I will take a look tomorrow to see what is feasible. Thanks so much.