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The hum you hear is the air pump I believe. Mine does it. Always has. And yes, there is a buzz too just before cranking, I think it is the fuel pump pressurizing the line.
I'd be suspicious of your coolant leak.
If the O2 sensor codes keep coming up, replace the sensor. Those things have a finite life span. Have you replaced them ever? I've replaced front and rear each once over the life of the car.
Your overall problem has a lot of similarities to the infamous "lawn mower syndrome" (LMS) well documented here on the 850. The main difference is your problem is happening with a hot motor, but LMS happens with a cold motor, driven just long enough to back the car from the garage to get the lawn mower out. It has happened to me on numerous occasions, so now I always drive around the block whenever I need to move the car.
I've been away from this forum for a while, but in earlier years, there was no known cure for LMS. I had various sensors replaced under warranty (many of which you've been naming) all with no effect.
My 850 has been doing the occasional fast idle recently, but no codes are getting set. A little stomp on the pedal and the idle gets right back to 700 rpm. I've got about as many miles on mine as you do on yours.
FWIW, my 850 has only died once while on the road, due to a dead battery. Once it gets started, it doesn't stop.
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'94 945t; '95 854 base
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