Ever since i got back from my trip to onterio the odd time when i turn the car off it sounds like its turning over with a weired hissing noise or it back fires sounding like a gunshot.

its a 1967 chevy II with a 250 w/ 2 speed powerglide

Maybe a loose spark plug?
Manifold leak?

i checked all the sparkplugs and there nice and tight i doo have a small exhaust manifold leak and i have some one looking for a new exhaust manifold for me so hope fully thats it.

It sounds like the engine might be dieseling.You may have the idle speed set to high and have carbon deposits built up in the head.You might want to check your idle speed and add a anti diesel solinoid.A water injector will get rid of the carbon deposits.Amsoil makes a product called Powerfoam that also will get rid of the carbon deposits.
EvilDr235