[s-cars] Whistling revisited

[Andrew B]

So I've had this problem before, but disregarded it as it didn't seem
to affect performance at all.  I met with another Scar owner last
night and realized it isn't normal at all....so here goes:

I have a distinct whistling noise above ~750RPM but before boost
actually kicks in. I can hear it most with the windows down cruising
around 2500-3000RPM. It sounds like it's coming from the turbo, or at
least that side of the engine. The car seems to make good boost, and I
haven't really noticed any weird symptoms. I initially ignored it
because when I had my head R/R'd, it came back to me with this noise.
I can't say with any certainty whether it was there before or not.

Any ideas on what it might be? I don't have the gear at home to
pressure test, but maybe I can check with windex or soap water to see
if any bubbles form. My lower IC hose is samco, but the upper one is
stock. IM and turbo are stock, 135k miles on all. Could it be the
bypass valve?  Can I test that myself?