Gradually introduce vague noise, or so it sounds (4-2)
I believe the answer is:
fade-in
'gradually introduce vague noise or' is the definition.
I can't judge whether this defines the answer.
'it sounds' is the wordplay.
I cannot really see how this works, but
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is found within the answer.
'sounds' could be 'din' (a din is a noisy disturbance) and 'din' is present in the leftover letters.
The remaining letters 'fa' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'so' is the link.
Can you help me to learn more?
(Other definitions for fade-in that I've seen before include "Introduce a picture gradually" , "Gradual increase in volume from zero" .)