Fuel pump or???

I have a 99 dx mt 4cyl with like 52500 miles. First of all, it has an exhast rattle that seems to be coming from my cat... any ideas? The other major problem I am having is my car is not wanting to start.. It takes a while to get it to fire, It turns over fine, just not fires, but as soon as it does it runs just fine. At first I was thinking it might just be a distributor cap with a crack that was letting mostiure get it, but then would dry up as soon as the engine was running, but today it was really really bad, then I went and filled up (I had been pretty low) and now its starting better, still slow, but much much better.. Is this possibly because of the fuel pump? if the fuel tank is a pressurized type, could it even be my gas cap not allowing very good pressurization or the vent on the cap could be plugged causing a issue? Im not sure and would liek to get it figured out before it totally goes

when car is not starting, go check for spark and fuel.

as far as the rattling goes, get under your car and look.

I did get under the car and look-thats how I know its coming from the cat, but it is like inside of it...

Check the spark plugs-they are fine, plus if it was a problem with spark then the car would be running bad after I start it as well.. but as soon as it starts its fine...Which is what is confusing me...