In turbocharger four-strokes diesel engines: what is the probable cause of mechanlcal noise ( knock ) in engine?

At the Diesel Engines knocking occurs due to wrong ignition timing of the fuel/air mixture . it is due to problem in mixture and ignition . it is also due to suddenly variation in power load. it also little effect due to jacket water temperature high. 

     If an engine is equipped with turbocharger the air is compressed to a higher pressure by turbocharger so that the temperature is also increased due to compressive heating. For this reason most diesel engines with turbocharger are equipped with after cooler. And if any restriction happened in heat exchanging of the after cooler, it would increase air temperature at the start of the compression stroke, which in turn raises all temperatures in the remaining cycle.

This can cause self-ignition and knocking problems in the latter part of compression or during combustion.  

 

if you have a knock in one cylinder check :

Excessive fuel .. bad injector

piston sieziure due to lack of oil and poor cooling system 

damage to valve train .. inspect cam lobs .. bent push rod .. valves 

excessive valve lash will make also noise sound

if the knock is common between many cylinders .. check :

engine timing 

Turbo charger .. affecting A/f Ratiio

fuel quality .. low cetain no.

lubrication 

Mechanical noise of a turbo charged diesel engine caused either by defective turbine shaft bearings or the valves having excess lash, or the defective crankshaft bearing and piston pins.   

Are you Refering to the surging from the Turbocharging ! the loud noise from the turbos is it what you are mentioning ? Because Knocking is somehing that exists for ever in Diesel Engines,

At the Diesel Engines knocking occurs due to wrong ignition timing of the fuel/air mixture before or after the ignition time, and not at the exact time when the piston is some t degrees before the TDC which is the right time to ignite the fuel/air mixture, and that cause noise of the engine, vibration, wear of its parts, and also much exhaust gases, and all of this cause of poor ignition 

Generally Knocking the engines means External ground forces affected to more than Engine Torque Ex : Up and downward direction of the road, To drive down ward direction of the vehicle, do not accelerate,when accelerate the vehicle, RPM more than engine output ie,weakness, tapperness, oilty,broken the Engine block etc