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The AOS has two different modes of failure.. Based on the fact that you had vacuum on the oil cap and the hissing went away when the cap was removed, I'd say the AOS has failed in mode #2.
When this happens the engine can be compromised as the extreme crankcase vacuum pulls oil from the oil pick up tube, reducing oil pressure at idle and low speeds..
I just had this happen to me on a test drive a few weeks ago, the event almost cost me my life when the AOS failed and oil vapors lit up behind the car like Haley's Comet...
When the AOS fails with method 2 you need to ensure the front and rear engine seals have not been sucked into their bores by the vacuum too deeply and that the oil fill tube wasn't broken when the vacuum sucked it flat..
Did your oil pressure light illuminate?
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