BMW Repair: Injectors/crank shaft, crank shaft, bmw318i

Question
Hello,
I bought a 1999 BMW318i three months ago.Two days after purchase i noticed a slight tapping noise coming from the engine.Over the last few months it has got gradually worse until today when it became unbearable and i felt i was doing damage to the car.There is a constant tapping when idle and very noisy when reved.I spoke to a mechanic just after buying and he believed it was the injectors tapping on the crank.
Is this a major job to replace crank and injectors?
Is there anything else that might be causing the problem?
Would you have an estimate on repair? time/cost

Answer
The information has been confused somewhere.

The injectors can't tap on the crank....they are in different areas of the engine.

Tapping is often associated with valves and the camshaft.

If it is the crank, it is a very major job to replace.  Usually replacing the engine is a cheaper option.

Take it to a mechanic who knows BMW's well, and get a good opinion on the situation.