Hey my Monte's been acting up lately. When driving and when you get up to 2000 RPM it makes a steely buzzsaw kind of noise. It even does this when you rev to just 2000. I know the noise is deffinitly coming from underneath the car, so it's not the engine and I know its not the trans. I just can't seem to pin-point it. Also I need a new cat, so mabye that's it? Please help!

 

You mention needing to replace the cat converter...are any of the baffles loose in it? Maybe when you increase rpm one is 'flapping' in the exhaust stream causing the 'buzzsaw' sound you're hearing?

 

If it does it in park and while rev-ing the engine, I would look at the resinator (those tend to rot out and pieces of metal flap at higher RPM). My Monte did it and long story shot, I had it replaced with a piece of pipe.
My wife's Impala is just now starting with this issue.