My Rats make usual noises

Question
I have 3 Rat's all female. The two younger female's 3 months old make a continuous, "Grumble, Grumble" sound, when I hold them. Not a squeak nor a peep sound, although they do that, too. What does that sound mean, are they content, or in distress? Thanks!!!  

Answer
They could be congested.  When they are content they grind their teeth which is known as "bruxing" and their eyes vibrate, which is known as "boggling"

I cant really be certain what is going on without hearing the noise though.  Its just too difficult to diagnose this way.