Oh dear!! I have read that when they are unhappy...
Sounds Your pig will make a fairly loud 'brrrrrr' noise - if you are holding your pig at the time you will feel her vibrate (not to be confused with the softer happy purr of a happy pig).
You will often hear this if they have heard something they don't like (in my house this happens when someone rings my very fierce doorbell).
That is what mine have been doing!
( I have to say though...my house is very noisy with my squealer of a DD2! She squeals loudly at EVERY opportunity! The Guinea pigs made her squeal quite a lot!!!)
Well they had better get used to it because the kids are not going to leave them alone any time soon, lol!!!