Rumble When Wind Blows

We have a very nice standing seam metal roof that was installed about 1 year ago. It is properly installed, flashed, vented and we know of no problems with it. However, when the wind blows about 20 mph, as it is doing now, we can hear the roof rumble. There is no banging sound like a panel is loose, but an audible rumble. Is this normal for a standing seam metal roof, or is this an indicator that the roof might not be attached completely? We cannot see any movement from either outside or inside in the attic. We did install the metal roof over 1x4s which were installed over the plywood deck to provide an air space, and that could provide an airspace for uplift of the roof. There was also some question as to nailing the 1x4s to the rafters without being able to see the rafters due to the deck, but our inspection from the attic side thought that most nails did hit the rafters. So, we can think of two reasons why there could be a rumble. But, we don't know if other metal roofs can rumble during windstorms and be adequately secure. Do you have any ideas or opinions?