Jackaroo injectors

I have had a 1994 Jackaroo V6 petrol motor rebuilt recently. It has a harsh knocking noise that the mechanic reccons is normal & the noise is injector noise. The motor didn`t have the noise prior to rebuild. It sounds like a desiel motor ! What questions can I ask the mechanic to proove that the noise isn`t a lifter? He has put heavier oil to hide the fault but it`s still there. The mechanic is comming up with excuses every time I tell him im not satisfied with the noise that wasn`t in the motor prior to rebuild.