The Code 25 makes a strange sound when letting a plucked guitar string sound die out, only when "amp" is selected (might be digital artifacting?) I saw one question about this via USB, but what I hear is not via USB, just out of the amps own speaker, no USB connected!
When using ‘Amp’ only (with no other buttons selected), the general sound might be quite fizzy. This is because the concept is to use one of these preamp sounds in conjunction with a power amp and cab model.
Even so, when holding the notes out, the sound should just decay naturally. If it’s not, there’s a high chance that the noise gate is set high and interrupting the signal, so reduce the gate down to zero and try again (this is also accessed in the amp menu).
If it’s still making the noise, a rough recording and settings information would be helpful for us to replicate the problem.