I thought it was related to XP X64, so I tried a test install of X86 with the same problem.
I finally figured it out.
I noticed several things about this problem.
1) When ever the sound would start to buzz, I would get micro-stutters on the screen.
2) I would only hear the buzzing when I was in the cockpit. (2d or virtual, didn't matter)
3) I would only hear the buzz when I was able to see the ground, or the clouds. The more I could see of either, the worse it got.
So, I changed sound cards. No good.
I ran DXdiag.exe. All checked out fine.
I updated drivers. No change.
I updated DirectX. Nothing.
I turnd down my soundcard's hardware acceleration, and noticed that with it all the way off, I didn't get the buzzing anymore, but during the times I would normally hear the buzzing, I got no sound at all.
So since I knew the issue was limited to FS9 only, I booted the sim, and as a last ditch effort, started messing around with my sliders. After a little trial and error, I discovered that it was just because I had my target framerate slider set to unlimited. Set it to 90fps, and it got better. 80fps, better still. 75fps and the issue is no more.
Not sure if this will help anyone else, but after all the frustration and subsequent troubleshooting I ended up doing, I thought I'd post this and hopefully save someone else the headache.
Just one of those things that is so simple, I overlooked it.
