Hydraulic lifters like Ed says are known faults, but try this , give the engine a flush out with oil system cleaner first,drain oil out, and try using a 0w 30 grade full syn oil , in some cases the oil carnt releive fast enough from the lifters so they jack out and hold the valves open till the oil gets more fluid and lifters return to normal, 16valve peugeot petrols do similar. Its worth a try before a big expensive strip down, but advise customer it may not work.
hth. Lee