Whats that noise ??

I have been looking for a new car, namely a landrover discovery, I have seen a cheap 300 tdi that im interested in. When the engine was started it sounded a little tappety ?? Would it be the actual tappets I can here ? or would it more likely be something else. If I bought the vehicle and it was the tappets would it be a case of just adjusting the tappets, or is this a common fault leading to more expense ?????