Flapping rooftop at speed

My rubber roof is in excellent condition. Looks great, very pliable, little time in the sun. When cold it is pretty taught.

But on a reasonably warm day it flaps at speeds above roughly forty. I can hear it as I drive. I'm concerned this is a lot of stress on it where it secures, and it seems simply wrong that it is happening. Always looks good, no sign of trouble, but I can hear it flapping away.

What's normal?
 
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