I usually have to burp after a correct swallow and though I've been mewing for a while now, I haven't been able to improve on this. I am not totally sure where the air is coming from - mouth or airway.

Below is some advice I have found by searching but I still can't figure it out so any further help would be much appreciated.

This seems the most relevant:

Posted by: @shakti_om

@freddie @hamiddkhnn  It's hard to imagine belching being caused by nose breathing. I thought it was caused by swallowing air, which is a problem I have faced while learning a proper mew push swallow. One thing that has helped me, is after I have "pitched" the saliva passed by tongue and towards the back of my throat, I really make sure to push the back of my tongue up when swallowing. This seems to prevent any air from going down.

Source: /community/community/main-forum/why-am-i-burping-with-nose-breathing/#post-16871

Another relevant thread: /community/community/main-forum/swallowing-extra-air-help/#post-17001