I used to have a friend who drummed on everything. All the time. There was always a beat apparently running through his head that no one else could hear. At first, you just wanted to yell, "Sit still!" but over time, you got used to the constant activity. And hands thumping on tables, legs or dashboards weren't loud enough to be confused actual drums, right?
Eventually I even came to appreciate the rhythmic background of actual drumming, instead of something like the random vibration that tends to bounce my leg endlessly. Random bouncing is much more chaotic than a real beat, even if you can't hear it.