I had this for a very long time and in my case it causes nausea. It was decades ago but I still get it from time to time. Back then they put me on every acid blocker there was and that never helped. What did help was simethicone (ingredient in Gas-X.) I would give that a try and see if it works, and that could help you narrow it down. Or at least get relief.
In my case the endoscope found that my stomach lining had no folds, but they had no idea the cause. Just recently, about 30 years later, I was diagnosed with an inborn immune deficiency and this is a potential autoimmune side effect of that where the body attacks the stomach lining. This is not to say that is your cause, but I consider it a miracle that some random surgeon suggested the simethicone after a surgery, when no other doctor ever has.
With the simethicone, you take it and don’t think anything is happening. Then in about 10-15 minutes get up and walk around and you will belch (a lot) and it will be over.