Ventilation vents

I presume wagons are the same but my ute has a bit of a problem.
When windows are down, one of the vents (picture attached) flaps and makes a very, very, very annoying noise and when windows are up, no noise.
There is a slotted flap inside that is supposed to let air out when internal pressure is higher and stop dust and water entering when external pressure is higher.
How do I stop the flapping noise without blocking it off with tape as I have done on the left side to shut it up. Worse that ex wife.Attachment 85939
I have taken outer vent off and it looks right but obviously it is not.

maybe open it up and see whats lose inside.