Me too initially. I was quite surprised at the airflow noise once I had fitted it all. I thought I had made the wrong decision to use solid ducting.
But from what I have read since, the hard surface of the tubing can reflect a lot of the sound (created by high airflow) down the tubing. Noise is also transmitted more readily through the material itself (any vibration noise etc), whereas with the flexible ducting, the flexible nature dissipates a lot of the airflow noise and any physical vibration. The ribbed nature of the flexible ducting introduces an uneven surface (like a basic baffle) which again seems to dissipate airflow noise.
I actually tested this. The fan sits in-between two sections of pipe. I removed one and tried the flexible ducting. The airflow noise was noticeably reduced. With the solid ducting there was a whistling/rush of air noise along with a slight hollow/echo reverberation sound.
As I preferred the rigid ducting, I decided to use some makeshift baffles/mufflers. I used some pipe insulation which was 100mm in diameter and had an internal bore of 80mm. I slotted this into the rigid ducting so it lined the inside perfectly. It really worked to dull the noise. Yes, the airflow will be reduced somewhat but it is now much quieter, better insulated (internally and externally).
The resistance of the grill still introduces some noise. There are low resistance options. But, again, I prefer the mesh dust and fly filter.
I would imagine a good solution to reduce some noise is to have a small amount of flexible ducting joining the in-line fan to the solid ducting.
Perhaps I also underestimated the power of the fan (even on the lowest setting) for my requirements. A less powerful fan may have a low enough airflow (but still the required amount needed) to make the problem of airflow noise a non-issue. However, this was one of the lowest power but good quality in-line fans I could find.
