Hmmm hard to know really. Guess its how long you can put up with the noise. An injector that is out of spec due to wear etc can over fuel on one or more cylinders and potentialy cause damage to piston etc. it over heats the crown which can crack or melt and blow a hole in it. Obviously not good. Diesel knock is premature ignition of the fuel just like pinging in a petrol and does similar damage. Its all about air fuel ratio and atomisation and a bunch of other things i dont really understand! I would first try at least a liquid moly purge, maybe two and a ecu reset. Could just be the injectors need a real good clean. If they get some crud in them they can dribble rather than spray with a similar effect. Our common rail setup is super duper sensitive to any foreign matter at all. Remember the ecu is programmed to inject a certain amount of fuel depending on demand, air available and so on. The amount that actually goes in to the cylinder is down to how accurate the injector has stayed since it was made or at least last measured. In reality the expected injection quantity and the real quantity are potentially quite different which stuffs up the ratio.
At only 155k i wouldnt jump to the conclusion that it is an injector. They seem to be able to do around 180 to 200k in alot of cases. Having said that mine are totally shot at 178k! And i have heard of them going as low as 100k. Luck of the draw or fuel quality or manufacturing tolerance or driver behaviour, who knows! I do know any water in the fuel will quickly damage injectors. Water at these pressures is an incredible cutting tool.
Over all if its a ticking i wouldnt worry about it. If its a loud and embarrassing rattle it could be injectors