I cleaned the oil cooler with de-greaser preatty good and spreyed it down with water. It was mostly dust and bits of crap stuck in the fins. The radiator was preatty dirty too. I did the same with that too. I put it all back and ran the 743 and the same thing agine. This time i payed more attention to what was going on. After about 8 min ( its hot out side today) of working it started to really make that load noise. Another thing i noticed is when you first start driving it makes that noise at low rpm but if you turn it up it goes away kind of. Its so loud it even drowns out the sound of the motor and 1/2 throtel. I try to turn right 180 degrees and it makes the sound but when you turn left wow is it loud and i almost have to pull the stick all the way back and it still hardly does not want turn. And when i look at the left hand side wheels they are hardly turning they just kind of skid along unlike the right wich skids and turns fine. I also feel the left hand stick kind of vibrate more while i try to turn and its harder to pull on. Another thing i obseerved was that it does not drive straight. The left lags. I hope there is a solution, i am sure there is and all it will is take time to find
Its normal for it to make a lot of growling at low rpm, you will feel a lot of vibration in the sticks too. At mid to full throttle it should be smooth and little noise should be heard from the hydrostatics unless you push in to a pile of dirt.
One last thing i would try before attacking the pump would be to swap the hydro motors over. If the problem goes from one side to the other it soulds like a motor problem. If not, it could be the pump(s). Your hydraulics still operate normally? like lift and tilt, no noise while siting still? It will growl a bit if the RPM is low, thats normal. But at a good speed it should be quiet and quite responsive.
You may have to take it to the dealer to get a test done on the pumps. They can do a flow and pressure test that will tell you if the pumps are ok or not. Not something you wanted to hear i'm sure, but it may be needed yet.
You did mention you bought a new motor for the LHS so it does sound like a pump problem. The problem is your main hydraulic pump acts as a charge pump for the 2 pumps for the left and right side drive. If its weak, there may not be enough charge pressure for the other pumps that will cause a loss of drive. You could put a pressure gauge on the pump, rite at the back near the fuel tank. There is a pressure switch here, remove it and put a gauge in. The pressure from memory should be around 100PSI. My manual will give me an exact number. Block the loader and start it with the gauge attached. Try moving the sticks and see what the pressure reads. There shouldn't be buch of a drop as its not under any load. I will try and get time to do this to mine to compair pressure readings.
If i don't get back to you tonight, you can e-mail me. My address is in my profile, this way i can't forget to do it!