Flapping sound on metal roof

My son recently closed on a house where the seller installed a metal roof of 9" or 12" rib spacing. i.e. not a standing seam roof. The house is sold and the owner has no interest in being helpful in any way. We are trying to find out where they purchased to roof to pay them to come out and advise as to whether it was installed correctly or not.

Yesterday we were at the house and it was pretty windy from the North West. The front of the house faces South. We were inside and started hearing a loud fluttering sound. My wife commented that a helicopter was flying over because it sounded like one.

It ended up being wind related. Standing on the down wind side of the house, we clearly heard a fluttering noise coming from the roof. The roof didn't seen to be vibrating, rather it sounded like a membrane underneath of it was flapping. It was bad enough that I could not imagine trying to sleep inside of the house.

We went outside then found that they had screwed the tops of the ribs. The overlapped ribs were screwed every 16" or so and the middle ones were spaced every 32" or so. Sorry for inexact measurements. It seems that the roof itself is secured well enough.

Thoughts or ideas of what to check?
This is my sons first house and I'm less than thrilled at having to deal with a roofing problem. Hence, asking expert opinions.
Thanks
John