on my 94 Z28 at about 1500-2500 rpms it feels like the car is having a lot of trouble pulling itself. i also recently developed a oil leak and i am having a whistling noise either from my manifolds or valve covers, i cant tell which. ive already replaced the throttle position sensor and fuel filter but no luck. i was told that my cat could be clogged up. the car seems to breath fine at idle but it does seem a little more quite while running. also if it had a small clog and was forcing some air back through the exhaust then i asume that could cause the wistling noise. not saying for sure this is my problem but if anyone has any advice to give me about what could be wrong or the best way to fix it please help i need it bad.

 

I had this problem and power dropped off significantly. My car had a rattle noise in the cat converter. The insides of the coverter broke up into several pieces and I could hit the exhaust with a rubber hammer and hear the pieces inside the cat rattle around. I knocked out the material inside the cat and the problem was solved plus the exhaust sounds better.