If you haven't, you might try using a piece of vinyl tubing like a stethoscope to localize the hissing. My guess is that the IAC is cruded up and the air is hissing as it tries to get through the throttle until the engine warms up and the IAC opens a bit.

I didn't see "cleaned the IAC and throttle body" in your other list of repairs. Seems like no one ever does that and one out of ten posts is about rough idle. In the twenty-five-year span of Volvos with which I'm familiar, seems like the IAC has hardly changed at all ... you'd think by now there would be a little red flag that pops up when it's gummed up.

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'96 850T and '83 244 DL both at about 200K, '97 850T5; gone but not forgotten '81 245 and '64 Amazon