engine vibration

I have a 1994 Infinite J30. It looks like new and ran smoothly until I had my timing belt change in January. The water pump was changed at that time. Now the car has a vibration in the stearing wheel. I took it back to the mechanic who checked it out and told me that they could not find a cause for this. The vibration is coming from the engine. It must be the age of the car. However, this vibration did not occur until after the timing belt was changed. I have 114,000 miles on my car.