losing power

my 97 bravada was making a strange noise which ended up being my catalytic converter, which i havent changed yet but i am losing power and it is really sluggish going up hills and when it gets above 2500 rpm i get a strange whistling noise. could all of this be because of the converter or any other suggestions?

If the cat. converter is melting down, it can kinda clog up the exhaust which will raise your exhaust pressures, temperature and reduce the exhaust flow...kinda like running a foot race inhaling through your mouth and exhaling through one nostril (yep, that's how the dictionary says it's spelt).