It's not normal for a healthy rat to make snorty grunting sounds. However it's quite common for a rat with a respiratory infection to make snorty grunting sounds when stimulated, such as when they're out of the cage playing, or when being held.
Gwen I would be inclined to believe your instincts were right and Mutley does has a respiratory infection.
What antibiotic did you put him on? Was it baytril, or doxycycline? If yes what was the dose and how long was the course? In my experience most rats being treated for the first time will respond to baytril on its own or doxycycline on its own. However there will always be exceptions.
One of my rats is prone to myco flare-ups and has ongoing respiratory problems. The first time I treated him for the snorty pig sounds I used baytril, but it didn't work. I had to combine it with doxycycline to get rid of the sounds.
So Gwen I would recommend trying again but with a different antibiotic, or a combo.
Meanwhile Carina, why do you believe the noises your girl makes aren't respiratory? Is it because you've tried antibiotics with no improvement or because she seems otherwise fine?
The thing to remember with mycoplasmosis is it's a very gradual disease. Unless a secondary infection kicks in the rat will often seem fine with the only sign something is wrong being these grunty sounds. These sounds can continue for weeks or months with the rat appearing "fine", but damage is being done.
Thyme, I'd also recommend going back to your vet for a second opinion. Why did the vet think they were okay? Was it because he/she listened to the lungs and they were clear? If yes then you may need to point out that a rat with an
upper respiratory infection will have clear lungs.
I know some people will tell you their rat isn't sick, it's just a snorter or a grunter or a chirper, but until I actually own a rat who snorts, grunts or chirps and who doesn't respond to any of the recognised respiratory antibiotics (baytril, doxy, zithromax, clavamox etc) and who lives to a decent age and dies of a non respiratory related cause then I remain sceptical. In my experience, weird breathing sounds are never good.
So my advice to all three of you is to get your rats on antibiotics and see if that makes a difference.
Edited to add - does anyone's rats sound like the rat in this video?
http://www.youtube.com/watch?v=vLf8Umaj-Pk