Have been playing a Telecaster for about 9 months and am getting tired of the unwanted hissing, i've narrowed down the problem to the guitar as when i turn the volume on the guitar down the hissing stops, however i'm just wondering if a noise gate would help clean up the issue or if i should rewire the guitar (lot's of crackling when i use the toggle switch, the volume knob can only be turned down about 1/4 to half way down and the volume is completely gone the guitar)
a noise gate may cut down some on the hum but it won't eliminate it. you may want to check and make sure the grounding wire is properly attached as if it isn't then you will get noise. 60 cycle hum is just one of those things when dealing with single col pickups especially if you are using mre than a little gain. you can get something like a Hum-X that will eliminate the hum.
Hissing like white noise sounds more like rf noise than mains hum. Mains hum is a fixed pitch low note. The cure for rf noise is to shield the control and pickup cavites, and earth the shielding. Also make sure that the bridge plate is earthed.