Looks like they focused on air intake, but there are other sources of vacuum or air leaks - the vacuum system itself, MAF's, any sensor that measures air, etc. What's tricky about air leaks is that sometimes people have different perceptions of where the sounds come from because they hear things differently. The best thing to do is get a couple of friends over, pop the hood, fire up the engine, and get a consensus as to where the whistllng is originating.