How long have you had her?
Is she confident with you yet?
The most common reason is that she is scared. If it’s when she’s on your knee put a fleece on your lap, make her feel safe and secure, try feeding her some veg.
My boys did this for the first few weeks, I was actually upset, these are my first pigs and they hated me. They get over it though, and so did I. just keep making them feel safe and they will learn to trust you. The vibrating will stop.