No problem I said. So I head over and open up the computer and find that the client had unscrewed one of the fans attached to the inside of the case. These fans remove hot air from inside the computer.
The fan was loosely inside the case. I attached the fan, started up the computer...no vibration and all seemed to be well. In the back of my mind I thought "this is too simple". Anyway, I left. The next day, the client calls and says that the computer is moving across the floor again.
I head over and take a look. I open the computer and start it up. All seems well. No vibration. He says it usually does it when he goes to youtube. (hmm I think, first hint) Computer warms up, fans start working faster and vibration starts.
Yep, after a couple of minutes, a loud hum and vibration starts, then drops off. Starts again and drops off. Each time, the computer vibrates more violently. I have a look at the back of the case and there is a blade from the fan which cools the processor sitting on the bottom of the inside of the case. The missing blade makes the fan out of balance and ..voila! vibration.
Thinking back, maybe I could have picked this up the first time, but as we say especially in computer repair...youre always learning. Even after 20 years!