Mountain Bike Brake Noise

[Donquixote]

I'm experiencing a weird problem with the set of Elixir's I've just fitted to my Kona. I picked up a mint condition Kikapu - this thing was literally showroom condition - for peanuts and decided to upgrade it to use as an everyday bike. I replaced the old Marzocchis with Recon Silver TKs, and the Hayes mech. discs with Elixir 5s and G3 rotors (185 front and 160 rear). They work brilliantly; the stopping power and modulation is superb, and they've bedded in perfectly. But in the last week a weird humming noise has developed at the front - it's so loud that I could hear it even when riding with my headphones in! I presumed it was the disc rubbing, but it's not. The wheel spins freely and there is no noise when depressing the lever; I've cleaned the calliper and disc thoroughly just in case, and they're absolutely fine. It has been driving me mad trying to work out where the noise is coming from!!!
If I hold the bike upright and rock it gently from side to side, without turning the wheels and without activating the brakes, after a few moments the noise starts up... you know the humming noise that you get when you run a wet finger around the top of a wine glass? It's like that! When I hold the bike still again the nice continues for a few seconds and then stops. But when I hold the front brake on and rock the bike, it doesn't happen. All I can think is that it has something to do with the brake line, rather than the calliper/rotor - maybe air moving along the hose?

Please, if anyone knows what might be causing this noise TELL ME!!! It's driving me insane!!!!

The Kikapu's a 2007 model with a really tinny sounding 7005 frame. Could frame resonance be a factor?

DonQ