I first heard the expression from my cousin in Tynset.

It’s not the å ja… But the “Oslovian gasp” as Dr Skinnoven calls it in our cultural anthropology class here at UIO. Granted, many use it around Norway but his research has found it used and adopted by Oslovians and those from or transplanted in Oslo.
So while it means the same as å ja one takes in a gasp of air while uttering it.
I have become accustomed to it and even have caught myself doing it. His research over Oslovian speech patterns, which differ from much of Norway, feels it’s a mix of learned behavior, as well as patterns of speech those in Oslo affect in order to be distinguished from others.
My son is working in Hamar at some museums and he’s noticed that locals will poke fun at the “gasp/hja” from Oslo commuters.

Isn’t it fascinating how words and usage are exchanged and persist (or not)?! When I lived in Norway the ‘hja’ sound was so subconsciously familiar that it took me some time to be aware of it and there seems to me to be a correlation with the ‘aye’ I knew in my childhood home in North Uist. This charming ‘feature’ is not present where I now live in the Netherlands.