After a CAT, and Header install, I noticed a buzzing, like a marble on metal right at 4200 RPM. Heard better upon deceleration around 4300 RPM.
My distributor also makes a little noise when I start the car, but faintly only for a few minutes.

Any ideas?
Let me guess, crank up the stereo?

 

Heat shield maybe? It's hard to explain those quirky noises isn't it?? I developed a sound that could be described as a marble on metal after replacing the clutch that I think is the cat heat shield--which I took off and put back on with new hardware in the process. Sure is annoying though!

 

I'm having the same problem. I hear mine mostly at 3000rpm but sometimes it happens sooner or later. I'm convinced it is the heat shield on the cat, and it's really annoying. Time to re-tighten it I guess.

 

I'll be removing my heatshiels today and let you know.

 

Beware that it gets pretty damn hot w/o the shields in.

 

I plan to replace them with some ThermoTec goodness.