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.