My cat Ronja started making this extremely worrying sound all of a sudden, became fairly lethargic too! Due to this happening late in the evening I couldn't rush her to the vet. She was fairly calm yet alert, but only wanted to lie around, not eat even her favorite food. It got somewhat better, but still raspy breath and too calm after a short while, but that lasted for about 4 hours before it cleared up seemingly completely. First thing she did when it cleared up was go eat. Now I can't hear a thing even if I press my ear to her belly.

In the beginning she made a strange loud burping sound 3 times with a few minutes a part. A loud boom of a burp. She and her two longfurred buddies shed a lot lately, so not sure if it is hairball related (she did a few regurgitating attempts at first, but nothing came up)

Though the fit itself wasn't deadly, it turned out to be Lung Cancer. She lived a fairly normal life with very few instances like this, but you could hear her breathing was a bit struggled near the end. She passed away in my arms about half a year later. Her passing was quick and sudden, so she got to die naturally at home.