PetroPower: First you ask what creates pressure in a noncompressible fluid, then you state fluids are compressible, which is true. Excluding aeration that can lead to cavitation or to diesel effect, I assume, for consistency, that you ask what creates pressure when the threshold of compressibility of a fluid is already achieved.

Gravity creates pressure in a hydraulic system, output work of a PD pump creates pressure if it's not obstructed, but you know these and you want some other answer.

When increasing pressure is applied to a confined fluid using, let's say a PD pump, the translational movement of the molecules is progressively restrained up to zero which is probably the threshold of compressibility for that particular fluid. If pressure continues to raise, the vibrational movement of molecules is slightly affected, possibly leading to a phase transformation from fluid to solid. The measure and expression of these energies (translational and vibrational) is called heat.

On the other hand, heat applied to a confined fluid creates pressure. Thermal dilatation of fluids can generate enormous pressures that can crack accumulators, cylinders, hoses, etc. For this reason, when designing or troubleshooting a hydraulic system, heat is traced throughly. That's why some systems have heat exchangers and faulty components are identified with IR cameras or IR thermometers and hydraulic fluid grade is carefully chosen according to the environmental temperature in order to preserve its viscosity and lubrication property.

So again, my answer to your question is HEAT.

Regards,

Michael