My father had a surgery 10 years ago at the lower back for herniated disc. why is his left pulp vibrating non stop?

Vertebral discs: There are discs in between the bones of the spine. Sometimes they can be injured. They are similar to a tough rubber or plastic material. On the inside, there is a softer material (pulp) which can herniate out (squeeze out) when the disc is torn. A great deal of force is needed to do this- like a car accident, lifting something very heavy, playing certain sports are examples.