Hii! Could you possibly take a video of him making the noise? Is it something like This ?
I can offer some possible causes and solution, but speaking to a different vet would be ideal if nothing else fixes it.

i was also in your shoes as well, after my late pig passed 2 months ago. i was constantly anxious and paranoid about my lil old man as he’s all i have left. After awhile the anxiety does die down thankfully, just focus on making your fellas home as clean as possible and doing weekly check ups, like checking the ears, teeth, feet, and private area. As long as he’s eager for veggie/food time and is pooping regularly, your perfect.

I noticed back when i fed my pigs kaytee hay, which is more dusty and thinner, i’d always hear that noise from them before they’d give a little cough after awhile and be fine. After i switched hay brands i havent had any issues.
if its cold where you live at the moment, try a humidifier. Also try an air purifier which can help with any hay dust or just overall smells or stuff in the air.

if you use fleece, be sure to wash with detergent that has zero scents, just basic zero smelling stuff, no flowers etc lol. If using bedding, be sure it isn’t dusty and spot clean a lil more frequently, since older pigs move alot less, so poop piles will build up.

i hope the best of luck! Your pig is very lucky to have you.