I think I know what you mean. It's not a grunting noise like the others have described, it's like a breathing lung noise. My rabbit does this too when I pick her up or she's over exerted.. it's hard to explain. I was thinking of bringing her to the vet too. It's not normal sound but yet she...
Yes, I do see that it's plastic. I agree with you that it keeps them dry away from their urine/poop and that's extremely important but there should be an area that does not have grating of any kind - the rabbits will prefer it when they are chilling out. It should not be hard flooring...