The system is pressurised, it hasn't been worked on in many years, so the only place the bubbles can come from is corrosion, isn't it? I keep hearing them splishing around in the system, and I keep trapping them in the bathroom towel rail (the highest point in the system) but they keep coming?
Or you've got a leak?
You could try that leak stopping additive, not sure what it's called but my plumber used it to seal a small leak in my system - worked a treat.
If it’s corrosion it will be hydrogen. Pop a lighter next to and see what happens (watch the eye brows) or catch into a glass then burn. If so put some inhibitor in the system
If it was a leak, I’d presume the system would lose pressure over time
Does sound like a small leak somewhere rather than corrosion. Not necessarily a big deal unless it's a lot of air getting in and the system keeps needing repressurising....
There's 1.5 bar over atmospheric pressure in there. Air isn't leaking into the system is it?
If it was a leak,
Check the water tank for green and white veg with a smell of onions.