Belt Driven pto pump/grindingnoise when below 0C

[boltz]

Don't know much about hydraulics. Have an under hood belt driven Muncie pto on a truck. When ever the temperature drops below 0 C the pump and lines sound like a coffee grinder in pain. If you leave it running eventually (after 5-10 minutes, depending on ambient temperature) noise is almost completely gone. In summer noise is just normal hum. Have changed fluid and spin on filter and no difference. Hydraulic controls operated slower in cold and faster after warmed up, or to me they are normal and not affected by noise. Even the hydraulic fluid reservoir seems to have the sound like it is transfering throughout the system. Hydraulic cylinders do not seem to judder or transfer noise. Any suggestions would be appreciated, cause I have another truck with same system and nowhere near the noise. Thx for replies.:confused3:

 

[J_J]

It sounds like water in the fluid, and hyd fluid with water emulsified does not flow well in cold climates until warm.

You say you have changed fluid, but did you get the correct fluid for your climate.

 

[boltz]

According to supplier of unit it is the correct fluid and viscosity. I know what you are saying about water and fluid, and was thinking about changing it one more time since we did it last spring. If I tried some of the test paste that we used to use to check for water in our fuel tanks ( it used to turn purple when water present in gas), I wonder if that would be a valid test for water contamination in Hydraulic fluid? The unit that the pto drives is on a tow truck, and runs boom up and down and in and out, and wheel lift tilt and up and down. If it matters, this unit sat a lot, (2003 model ) and was used very little. Fluid that we originally drained out looked really clean. However, did not get all the fluid out of cylinders, just drained the tank. Would that have been enough to make a difference?