Throat Noise

Question
My computer is not letting me on the link you suggested.  It is a different sound other than grinding his teeth.  I have heard him grind his teeth before.  The sound is like a small motor humming.  Thank you

Answer

The grinding teeth is called bruxing, which I already assumed you knew that, but the other sound you describe is probable congestion. Rats are animals that only make one sound meant for us to hear which is squeaking . Other sounds, such as sounds of distress or sounds of mating are inaudible to the human ear. Any other sounds besides the grinding of their teeth when content/happy or sometimes when they are very angry or very sick (chattering) but any other sound, from a pig or monkey sound to a motor running, like so many people describe, is almost always congestion, nasal or possibly chest.  

Other wise, is he sneezing at all? Breathing fast?

Do you have a vet that is skilled with exotics and rats?

And your welcome, btw....