I just wouldn't ever wear ear buds while biking, even in car-free areas. I ride in NYC so getting rid of ear buds was an early decision for me as you can imagine. Now if I'm on a long/long-ish trip I have my Xmini capsule II speaker and mp3 player in a cheap top tube bag (roswheel, a fake chinese company) and that way the music comes right at me without distracting me.

I know exactly what you mean by that "guitar string" sound, and I personally go insane from it. I had the same thing when jogging or running on the treadmill. You also have to worry about the buds getting yanked out of your ears and flagellating around and getting stuck in you spokes, very unsafe.

Lastly, I read a study a while back that wearing ear buds while doing exercise is just flat out horrible for your ears because when you do exercises like running or cycling, there is less blood in your ears because the body moves it to your legs so your ears get even more vulnerable to permanent damage.