That is from air escaping the rear of the hood. There are many other car makers that this has happened to and owners usually add some tubular weather stripping to the back firewall where the hood meets it. They also add some vinyl tubing in the weather stripping to add some rigidity. The success...
If Lexus hasn't resolved the hood flutter issue by the time we get our GX, will be installing some sound deadening material on the underside of the hood sheet metal behind the factory hood insulation. It is hard for me to say what a good fix would be as I don't have our GX yet and can not study...