Hi Andreas, funnily enough when I was out there today too, one of the setups was also whistling whilst the other was not, on new 3.0 Fuselage/mast, but it was on a 78 fuselage, not really sure why this happens on only some and not others - but we did manage to eliminate most of that when using some light water paper on the back wings.
The shorter fuselage has the front wing closer to the mast. Most of the time you want to try to have the mast under your back foot and the front wing between both feet for the best balance. So when using the short fuselage and 1500, put the foil rather back from the centre of the boxes. When you use the longer fuselage, you can actually go even further back then, to get the front wing between your feet, 2/3 of the way to the back.
One thing which is clear is that Walchensee on a very light wind day, requires the foil a bit further fwd in total, vs a windy day at other lakes/places, as you lack the lift otherwise to keep going thru the big holes. So I even had to use the foil further fwd than Silvaplana, where we were racing at top speed with big wings for example.
Generally it is a good idea to keep experimenting until you find the ideal setup as it is a bit personal, also the difference between straight line riding and jumping is huge, on where you want to have the foil/front foot pressure for pop/release.