I've read the previous posts on these codes. I'm wondering about the fuel tank cap. Is it supposed to be airtight? When I shut the engine off, I can hear a hiss of air flow at the gas cap? Is that normal? Is that the unmetered air leak I'm looking for?

A leaky gas cap will have a different error message and totally unrelated to intake unmetered air.

You could try the 'cheap' route and clean the MAF with a spray cleaner, which usually last a few months.

Depending on the age and mileage of your car, you may also start thinking about a front O2 sensor, vacuum leaks around the manifold, as well as replacing the MAF.
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Keeping it running is better than buying new

The Volvo Service Dept. looked at this one, and found that the car had a PCV clog. The symptom is positive pressure buildup in the crankcase. You can test for this condition by sealing a latex glove with a rubber band over the oil filler opening (with filler cap removed). If the glove inflates like a balloon, there is a clogged or dysfunctional PCV ventilation. There should be a weak negative pressure at the oil filler port (you must be careful during the glove test not to let the glove be "inhaled" into the engine!)

BTW, a PCV failure that allows pressure to build up in the crankcase, if not corrected, can eventually result in the oil seals blowing out inside the engine....the repair can cost more than the car is worth. It's well worth the money (or time) to do a PCV cleanout. As a preventative, this would be done at around 90K miles.

We took the car to an independent repair shop to have the PCV breather box replaced. At the same time (90K miles), spark plugs replaced and throttle body cleaned. A repeat of the glove test showed that the PCV problem was fixed.

It's been 2-weeks , and we haven't had any recurrence of the engine codes. There is still a small shudder when stopped in "R" gear. The shudder while in "D" is gone.