I am having the exact same issue with my 99 S70. I figured I was looking at a clogged PCV system. I cleaned the MAF but the symptoms are still there. Let me know if you find a remedy.

We just had these put on our '98 V70 XC. They were the loudest tires I have ever heard. It was bizarre. It sounded like a prop plane going down the road. We took them back. The tire place said that they had never had anyone complain about the noise, but once the new tires were on it was much quieter ...