hi
my 95 rough idles until it warms up. I recently had the muffler replaced (most of the pipes including the converter)
I thought the noise would go away but has not.
it just turned 100K and had a tune up last summer.
it runs fine except for a hissing noise that pops up at times which i have no idea what may cause it
what can i suspect?

you probably have an exhaust leak somewhere. that is the cause of a lot of those kind of noises (pops).

Is the check engine light on? It might be a bad O2 sensor.

hey wat up, i wanted 2 ask u something..is it possible 2 put a bigger engine on a altima?..getting tried of the little 2.4

hissing noise?? makes me think vacuum leak.check all hose connections.sometimes a little 2+2/carb cleaner helps with locating a vacuum leak

hey wat up, i wanted 2 ask u something..is it possible 2 put a bigger engine on a altima?..getting tried of the little 2.4

I don't think the engine bay/mounts can support anyhting else. You could jsut go smaller with some boost. Highly doubt it though.