Abby seems like she has a wonderful owner.
Like you said, teeth chattering is usually a feeling of anger or disliking being where they are or what they are doing.
But, sometimes, my guinea pig Cherry seems like she is chattering, but really, she is just doing a sort of chewing, which a specialist told me is normal for guinea pigs.
When she is in your lap, see if Abby is making a circular motion with her mouth. If she is, it's just a habit that you can't get rid of, but it is not hurtful either. If Abby is chattering her teeth, maybe she doesn't like cuddling. Guinea pigs are not known to be cuddle animals like cats or dogs.
Cherry doesn't like cuddles much unless I am feeding her lettuce and petting her at the same time. Try to see what works with Abby and what agreements you can make.
Hope I helped and best wishes to you and your piggies,
~Savvy Cavy
