Well Tazza, not having any experience with a 743, I can only rely on the years of aircraft hydraulics which is pretty close to the way our machines are designed. That being said take all of this as information and bear with me because I know you probably know some of this. Humming sounds almost always come from something that is moving all the time and is actually a vibration of some sort. So looking at the way things work in the flow schematics of my 843 (and I think all the bobcats) while running and when every thing is sitting still and all controls in neutral, the only thing running all the time are the two pumps and gear pump. All the hydraulic fluid, while even under presssure, is just running in a circle. We are looking at constant volume pumps (front & rear) that supply hydraulic pressure to the motors that drive the wheels. And on the front of the pumps is the gear pump that supplies the pressure for the tilt & lift cylinders. The gear pump also supplies the hydraulic fliud to the inlet side of the front & rear pumps. When everything (levers & foot controls) is in neutral the shafts and variable pistion assemlbies in the hydro pumps, and the gears in the gear pump are the only things that are turning. The speed they rotate of course depends on the engine RPM. The only things that I can see that would or could produce a humming or vibration in there would be the thrush bearings or the shaft bearings in the hydraulic pumps. I lean more to the thrust bearings because when things are in neutral there are not any forces being applied to that bearing. The shaft bearings have the same forces applied to them all the time in neutral or not. Going back to the thrust bearings now, you said when you move the levers the humming stops but comes back when you go back to neutral. Looking at that action, we are moving the floating plate that applies pressure to the rotating variable piston assembly in the hydro pump. That action pushes or pulls the internal assembly a little that is being driven by the pump shaft and in turn is applying forces against the thrust bearing. That force against the thrust bearing stops the vibration until things go back to neutral. Bypass valves, port valves, relief valves, and the like are just that. Valves are either open or closed either manually or with designed spring pressure and thus will not vibrate. I know some people say that a valve will "vibrate" because it is opening and closing all the time very fast. That is not really the case and the noise one hears when a valve actuates is only the fluid under pressure traveling though it. All in all, I don't belive there is any wrong in the hydro pumps other than just a little wear and tear on someting that has to turn constantly by design. In my opinion it is not worth tearing it down to replace the thrust bearing unless you have to for another reason. Hope this provides a little more insight to it all.
