Our Tiger does the same. The pitch seems to increase in steps as you go faster. Our rudders and boards don't vibrate and I can't tell from listening where the sound is coming from, but I'd bet good money it is coming from the foils (I haven't bothered fairing them in ages and they are a little battle scarred from interfacing with wildlife while distance racing).

I'd concentrate on the foils - I think there is an article on the AHPC site with relevant information?

Chris.