I noticed this about a week ago: when I go down really long hills, I put the car in neutral to avoid excessive engine braking or wearing out the clutch throwout bearing. Since the engine is really quiet in neutral, I was able to hear a sort of buzzy sound coming from behind me. The other day I came home quickly to get something and left my car running in the driveway; when I got out, I could hear it, and tried to trace the noise. I ended up about halfway across the driver-side rear door. Something under there is making an odd sound whenever the motor is running.
My first suspicion is that it's some part of the exhaust system, but I have no idea what that is, and I'm getting new cam seals today, so I can't get up under there and look around at the moment.
I haven't noticed any slump in performance or economy, so if it is an exhaust issue, I don't think it's too serious. But man is it annoying. Any ideas on what it is and how it can be fixed?