Like everyone else’s comments, if it’s actually noisy you should speak to a doctor. But I do deep breathing sometimes when I’m angry/anxious to help me relax, and now my partner is conditioned to spot the very quiet sound of me doing it. I’ve had a chest cold for the last few weeks and he kept asking if I was okay because I was taking deep breaths.

My point is, even if their breathing is quiet, the sound of someone taking slow breaths can be noticeable. Only tip I can think of is to practice breathing smoothly and regularly. When I’m actively trying to take a slow, long breath I notice it can be quite uneven. I breathe in, that breath tails off and I breathe in some more. That leads to an uneven sound that is more noticeable. If I really focus I can breath steadily and smoothly and it’s much less noticeable.