What do these noises mean?

Question
I am a first time owner of two guinea pigs. One is very talkative. I have only had them for one month so I am still learning what each noise means. First, when we reach to pet her in her cage, she huddles in a tight ball and purrs loudly almost like a growl. Is this a good sound or a warning sound. We have already heard the teeth chattering before a bite sound when she attacked the other pig, so we know it's not that. It almost sounds like a car starting. Ha!
When we are holding her, often times she will squeak quietly and I believe she does that when she is calm and just wants to talk. But there are times when you walk by her cage and she squeals much louder. She has food, so I don't know what she's squealing about. Any ideas? Basically, I would just like to know if that grumbling when petting her in her cage is friendly and just an overview of meanings of all the levels of squeals. If you could give me some ideas, I could look for my pig's behavior at the time of sound. She makes me want to read her mind. She has a lot of personality and I look forward to learning about her more each day. Hope that you can help me. Thanks.

Jennifer

Answer
The sound that she is making is a calming/relaxing purr. They like that and she is just talking to the other guinea pig when you are holding her or just making noise
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