[Richard]

I developed a buzzing or whirring noise from the engine compartment that tracked engine RPM. At approximately the same time I noticed the engine running about 10 degrees cooler than normal and the fuel mileage dropped about O.5 mpg. 

Normally when the engine runs cool and the mileage drops I suspect the coil that opens the low speed circuit for the fan has died. The default position of the hi lo valve is hi. This is the valve and coil on the side of the brass block at lower left of engine. The hoses from the hyd pump enter the block and the hoses to and fro the fan motor enter the bottom of the block. 

So, the coil failed the first test. A steel washer wouldn't stick to it with the ignition on. It also failed a more scientific test when it ohmed as an open circuit. I replaced the coil. BTW, it is Eaton 300AA0095A MCSCJ012DW0B0010.

That didn't fix the cool engine or the buzz. I put a mechanics stethoscope on everything back there, and the loudest origin was at the hose ends for the fan. I found some obscure posts on hydraulic systems that mentioned that leaking valves can cause noise in hyd systems. I ordered and replaced the valve. It's a Vickers part SV-16-c-0-0.  I didn't take the old one out till I had the new one so I could do a quick swap and minimize fluid loss. The old valve would only open a small stroke. The problem was solved with the new valve. Buzz  is gone, and engine temps are back to normal. 

Hopefully this will help someone in the future.