I've recently upgraded to AT5 and i'm fairly sure the white noise wasn't there for the first day or two. It's really bad now! All of the presets have this constant white noise.
I saw someone else with this issue who was told it could be AT5 hasn't been authorised in the UK product manager, but i've checked and it's saying it's authorized.
When i click on the some of the items in the AT5 set up (the image of the amp, pedals etc at the bottom of AT5) it says "This feature is only available in AmpliTube 5" and i'm provided a link to "UPGRADE NOW". This link takes me to a web page selling AT5. Not sure if this means anything regarding the white noise, but thought it worth mentioning.