I noticed my Marshall Code 25 makes a hissing noise when I play through any of the clean amp models. This hissing noise is clearly audible when I play a note on the low E string. It reminds me of an old cassette tape. This is especially apparent with the cab emulation switched off. On the higher strings there is no problem. Is this normal? The hiss doesn't go away with the noise gate turned on. I have this problem with both firmware versions 1.44 and 1.45.
Just to check, have you tried another guitar or cable, what type of pick ups do you have.
It's hard to tell exactly the hiss you refer to. All amps have some levels of background hiss as long as its not too loud then you probably have to live with it. This will probably be more noticible if you put the Gate to zero and the hiss disappears as you increase the gate.
However I have noticed a level of "fuzz" on all sounds including clean tones. This is just digital artefacts that you get with most cheap modellers. I have noticed on the Code that the Power amp models seem to generate more of this artefacts. try switching off the power amp and see if that removed the hiss.
I have actually noticed that on the Codes clean sounds, the Power amp models dont really affect the sounds that much. On many of the clean tones I have actually switched the gate to zero and switched off the power amp modelling and adjusted the EQ to compensate and make it sound the same as the power amp model, and this gives a much more "clean" clean sound with a more natural decay.
Its possible some other part of the digital modelling may add some hiss too, so you could try narrowing it down by switching things off in turn and see if you can get rid of the hiss, and identify it that way.
Bear in mind though the Code is a cheap amp full of digital technology. By comparison I have a Vox AC30 C2X that cost well over £1000 and that hisses so much you can tell its switched on in the next room.
Or maybe there is something wrong with your amp?