the kitchen isn't fitted out yet and the valve is hanging from the duct in the ceiling (it's for an Island hood) with a large bag taped over it. The bag never fully inflates or deflates and as I said only extremely rarely with unusual wind directions will the flap clunk. I should add that I've the MVHR set towards positive pressure to safely deal with the not-yet fitted boiler stove, and that would make the valve even more sensitive...
this kinda thinghttps://www.amazon.co.uk/Klimapartner-100-line-Backdraught-Extractor/dp/B011INHF5A/ref=sr_1_14?dchild=1&keywords=passive+flap&qid=1627639391&rnid=1642204031&s=diy&sr=1-14
but with a thin smear of silicone over the central pivot.