I never realized that flexible ducting also acts as a muffler and reduces fan noise big-time.
Right now I'm set up with smooth ducting plus a can fan and it's a noisy bitch, even on the lowest speed.
Definitely got worse when I removed the phresh carbon filter to clean the sleeve.
For the next indoor grow I'll probably use a hybrid of smooth & flexible.
I'm re-engineering the space now in an effort to decrease light leaks -- not that it really matters for plants that were brought in from outside.
They tend to be way less finicky than plants that were started indoors. Light leaks don't bother them at all.