Mercedes C180 Clasic Auto 1996?

Recently we had the engine valves reground but since then, when starting from cold, we get a clanking noise, maybe just once but on occasions up to 9 times. The noise stops when the engine has warmed up. The noise seems to be at the front end in the engine.Do you think it could be a sticking Inlet Valve?
Our mechanic has had the car in and says he has gone over everything he can think of, I must say that when warm the performance is better than it was, but he still can`t identify the fault.