answer the following first:
1) whats your car's oil change etc habit and history ?? you seem to me a little ignorant to the fact that engine oil and other fluids have to be regularly checked, hence the reduced oil level as your mechanic pointed out.
2) are you having or ever had "check engine" light or "OIL" light on ??
3) tappet adjustment ever done ?...usually diesel-engine-like sound comes because of loose tappets.