There is also a high frequency almost 'buzzing' feeling which can not be heard, but rather felt through its feet. This is an expression of nerviousness and fear. All of my Guineas done this when they were new, as they were nervious about being held. It doesn't happen much anymore though and they will get used to you.
My girls make 4 types of rumbles/purrs
1. Buzzing - which is the fear thing, almost like very fast shaking, quivering.
2. Alarm Rumble - Where they hear a noise they don't like and one will give off an alarm call, which causes them all to freeze then run off.
3. Rumble Strutting - Almost like a dance, where one wiggles it's bum, moving from one foot to another. My Dominant female does this to keep the others in check and to assert herself. Like a display.
4. Purring - When scratching two of mine on the shoulders/back. It is indistinguisable from the Rumble Strutting sound above, but is constant rather than a short rumble like the alarm call.
-Carma