It sounds like you need to do a PCV service on the car. The non-turbos have a vacuum regulating diaphragm that is part of the oil trap (breather) box. Over time that diaphragm becomes fouled with carbon build-up, causing way too much vacuum to be applied to the crankcase. As one person described to me, it's sucking air in through the crankshaft and cam seals, causing them to play like a kazoo.
Here's what mine looked like inside:

When you pull up the dipstick or crack the oil filler cap open it give the incoming air another path to take so the squealing stops.
I believe that all those codes you're getting are due to the effects of this huge vacuum leak and not an indication of a faulty MAF, O2 sensor, etc.
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Current rides: 2005 Volvo S80 2.5T, 2003 Volvo V70 2.4NA, 1973 Volvo 1800ES (getting ever closer to road worthiness)
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