First time this happened was this morning. It's definitely speed-related; I tried varying fan speed (including turning if off), heat/AC, inside/out, various vents - no change. The only thing that causes it is exceeding 35-40 mph, when it begins, kind of low-pitched, then increasing in frequency as the speed increases. It really sounds to me as if the flapper vent that lets in outside air to mix with inside air is vibrating. I also notice that gusts hitting the car from the front (it's windy today) raise then lower the pitch. Once I slow below 35 mph, it dies out and goes away completely.

My Volvo mechanic guy is stymied and we're both puzzling over next steps. It's crazy to have to tear the dash down and start replacing stuff without a clue, but that's the only (expensive) solution I've got right now. Anyone who's experienced this and/or can put me on a better track would be warmly welcomed to weigh in about now.

I can be emailed too at (remove no spam): nospamk4iq@att.net