Welcome to Mercedes diesels!

Assuming good oil pressure, and no bearing damage, which is not too likely since the bearings are quite long-lived and rugged unless oil flow or pressure has been lost, it's possible you are hearing 'nailing' or combustion knock.

You are corrrect, #1 cylinder is in front, #5 is in back. A few quick diagnostic tests regardless which cylinder sounds noisy...

Grab a 14mm open wrench and a paper towel. Loosen the nut on top of the suspect injector that holds the fuel injector tubing. This will create a pressure leak and that cylinder will not 'fire.' If the noise decreases, you have confirmed that #1 is the source. You can operate the throttle linkage to rev the engine and listen for the nailing... When finished, re-tighten the nut, firmly to stop the leak but don't overdo it. Wipe up the diesel drool with the paper towel.

Go ahead and try other cylinders to be sure. It's like pulling the spark plug lead on a gasser to find a bad spark plug, just not nearly as dramatic since you don't get the high voltage shock from holding the spark wire!

You can also swap the suspect injector with another. If the nailing moves to the new cylinder, there is an injector problem and it might need to be cleaned and/or rebuilt.

Swapping injectors requires a 27mm deep socket. The special injector socket is deep to avoid knocking off the fuel return line arms, but other sockets will work if deep enouth. You will also need a new heat shield for each injector, shaped like a conical crush washer and about $1-$2 a piece at your friendly dealer, and a torque wrench for tightening.

Oh, while you are ordering the heat shields, get one meter of braided fuel return hose, since your hose is probably old enough to crumble when you remove it from the injectors. One meter (3+feet) will replace all of the braided hose on your engine, a quick and easy job once the old hose is off of the injectors.

Before all of this, try the the pulldown Search feature, and look for 'nailing' which will give you hours of reading. Basically, it's a result of a problem with injectors such as a bad spray pattern, bad pressure, dirty injector, etc. that sounds like, well, a hammer hitting a nail into the block...

Sorry if I rambled...

Best Regards,
Jim