I have a John Deere 5105 with approximately 700 hours. When you are in idle after heating, I have noticed a fairly strong noise of “rattle, ratchet” that seems to come from somewhere on the left side of the engine. However, with only a slight load on the motor (that is, the connection of the power steering slightly rotating the steering wheel) the noise tends to disappear. I tried unsuccessfully the “stethoscope” method and tried to isolate the noise but I was not lucky. I have loosened the tension of the alternator strap but it has not taken effect either. I also examined the water pump, but there is no noise or leaks in the pump. Could it be a transmission gear of the injector pump in poor condition? How can I check it without removing the lid?
It sounds like a normal “noise” of the gears at idle. It is caused by the hydraulic pump when there is no hydraulic charge. When there is a hydraulic load, the noise of the game in the gears is silence. The distribution gear train has straight cut gears. If they were helicoidal cut, this would be much better. Unless you feel that the noise is getting worse. Then you may want to remove the lid to inspect the gears.
Before removing the lid, I would send an oil sample. If there is unusual wear on something, the results of the test would tell you.