I have to say, it's one of the stranger things I've witnessed. They make the same sort of sound that dogs do, but it's much quieter and there's no lolling tongue.
The first time I saw a cat pant was when Sarah (the wonderful pet sitter) came to drop off my keys. She also brought a feather wand that she said Melly had loved during her visits. I wasn't surprised, as it's no ordinary feather wand: it has a whippy handle that's three feet long attached to a string of equal length, with a large bundle of feathers at the end. Sarah gave a demonstration, using the flexible handle and long string to make the feathers fly around in a convincingly bird-like manner, and of course, Melly went bananas. Eventually she was sprawled out on the couch, panting hard.
These photos are from Sarah's visit and were taken by Geoffrey.
After watching Melly zip around the room in pursuit of the feathers, Scarlett eventually joined in. Since she is usually thoroughly unimpressed by feather wands (after all, she has caught a real bird), it was fun to see her batting at the toy and chomping the string.
That's me Scarlett's looking at. It was a very "Mooom, tell her it's my turn" sort of look.
Sarah generously allowed me to keep the toy, and I've been using it with the cats regularly. It's taken me a few tries to figure out how best to maneuver the feathers, but I had Melly panting today after about 20 minutes, so I think I have the hang of it! Next time Geoff is here I will ask him to take some video - it's hard to get the full, hilarious effect in still photos.