Heat Loss through bathroom ventilator - passive

When ever its windy I can hear the bathroom vent flap opening and closing. This is an obvious heat loss. I suppose all fans are prone to this. But I have a roof flap, an extra in line damper used for dryers and a Panasonic fan with a nearly useless flap. The pipe from the ventilator to the roof is only about 2.5 feet in length. I can't figure what to do to stop it other than removing the fan or replacing the ventilator. I have a window, so I don't need a fan. What can I do other than finding a "better" ventilator?
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