I noticed as my wife was driving our '04 XC90 5 cyl. turbo (231K miles) away (250 miles each way to Maine) this morning that there was a whining sound at idle. It seemed to disappear with acceleration. I had just added a little oil to top it up, and an oil change was done about 150p0 miles ago. 5W-30 synthetic every 5K miles. She arrived in Maine OK and said the car drove OK, but she noted that the noise was still there. I did a quick search and see that the issue may be something to do with the breather box. Can she safely drive the car home tomorrow, assuming this is the problem? Also, is this a necessary repair? The car runs well (except for this new whine), but the body is rusting and won't pass inspection. I can continue to drive it, and I would spend a couple hundred to keep it going, but as it's probably on last legs, I'd like to save money where I can.

Does the whining disappear if you crack the oil filler cap loose or maybe pull the dipstick out? If so then it’s likely something PCV related, as you already seem to suspect.

The hose that runs straight up from the breather box to the top of the engine is known to split open, just above the box, with old age. That could create a whistling sound during moments of high manifold vacuum.

Yes. I loosened the gas cap and then I pulled the dipstick out a little bit. Both actions made the noise stop. I’m wondering if this is a necessary repair. The car is old and, like I said, probably won’t pass inspection again, but I can keep driving it. Not sure whether to fix it or not.

If the car will not pass inspection, don't bother 'fixing' it. It is still drivable and not a concern for leaving anyone stranded.
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Keeping it running is better than buying new

Thank you! That's what I hoped the answer would be! :)