Noisy Extractor Ducting

[JamesK]

Hi There,

I've got a problem with a noisy Bosch Extractor fan. If memory serves it has a 150mm output, so I've had to use 2 reducers to get down to the 100mm ducting that is present in the kitchen.
I've created a video here:

The audio on the video isn't great. It sounds like something is obstructing or flapping around inside the ducting. If I remove the ducting the fan goes completely silent. Is the fact it's flexible ducting a cause of this?
Replacing the ducting wouldn't be an easy option for me, so I was wondering if there's anyway to make it quieter.
As I'm about to decorate the kitchen I thought I'd ask for advice here as the cover is off.
Many Thanks...

 

[foxhole]

Is there a non return flap on the top of the unit.? Once ducting is connected it creates turbulence which causes noise.Should be foil ducting above heat, plastic is soon destroyed by heat .