Finding boost leak?

[Michael Gough]

Hey guys,
I need some help. I have a large leak in the intake system, but only in
boost. The car runs fine in vacuum, in boost I can hear a large leak. It
isn't the turbo whistle that is supposed to be there. Then when I shift
there is a big woosh like a blow off valve would produce. The thing that
baffles me, I pressurized the intake plumbing, and I could not find a leak
even with 10psi! Anybody have any ideas?
Thanks
Mike