How bad is the tapping noise?
Any warning lights or oil lights on since this fault?
Have they ensured oil is getting to the top end,oil pressure test etc?
How many miles?
Yeah normally on 320d injectors can be a bit of common fail on them and can give exactly the same symptoms but also the engine would run rough/lumpy and you would notice a increase on fuelling consumption...i say above to make sure oil reaching the top end just in case..if it then you could rule out any possible issue as per oil - another issue that can be common on the 320d is the inlet swirl flaps..these have been known to break off and cause a similar issue from the engine..plus is parts of the flap gets ingested can cause damage to the engine..the other is as you say possible loose/worn valve lifters/followers..and these are relatively straight forward to replace.the car can be driven if it them at fault to a point..but will eventually need replaced at some stage..maybe try using a automotive stethoscope that would certainly help pin point the noise for sure//as per injectors or lifters issue...