Hissing on throttle tip-in; vacuum leak? AOS? Normal?

[Toddyus]

Had the engine cover off and there is a noticeable hiss on throttle tip-in. Sounds very much like a turbo blow-off valve. Is this normal? Could it be a vacuum leak, and if so, is there a common place to look? I also tested the AOS with a manometer. At temp, readings are about 5.25 in/H2O range and with revs hitting about 3k rpm (not at load), reached about 7.25 max. Seems normal range, but could it be the AOS?

Any insight will be appreciated.