Idle noise is largely injection timing. But engine age also has an effect. The earlier 4BD1 engines have looser clearances and that makes them knock more. Especially when cold. Later on (particularly after the 1988 update) piston/liner clearances got tighter and the engines got much smoother and quieter.
Retarding the injection timing a little will smooth them right out at idle, but make response a little duller and can make them hard to start in the cold if you go too far.


 
					
					 
				
				
				
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