OT? Noisy forklift hydraulics

I have an older (50's) Yale 4k lift with basic hydraulics - lift and tilt. Everything works fine except for a very loud buzzing sound when the revs are above about 2000 rpm. It is coming from the valve assembly. The threshold rpm decreases in cold weather and increases in hot weather. It does not go away when the hydraulics are operated but it does change pitch (lower) and intensity (also lower). Additionally, once the noise begins, it does not subside until the revs fall below about 1000.

So far, I've checked the fluid level and the pressure relief valve. I have increased and decreased the spring pressure on the relief valve with no discernible change.

I'm tempted to open up the valve assembly but having limited hydraulics experience and given the age of the machine (parts availability and having to deal with the dealer), I'm a bit leery. What else should I check? Anybody ever had a similar problem? Thanks.

-Chris

It was my experience back when I was repaired, dozers, loaders and the like. If the hydraulic pump is a vane type (housing about square) the pump chamber wears from the vanes and this causes the buzz. If you disassemble the pump the chamber should look like two overlapping circles, no bumps or sharp surfaces where the vanes ride. The vanes should be tight in the impeller.

Second cause is air leaking into the suction side of the pump, foaming of the oil is a sure sign of that. Shaft seal or hose clamp loose.

If the pump were worn, would the noise manifest in the valve assembly?

I would suspect a clogged screen on the hydraulic oil pick up line. I've seen screens get dirty and the pump will suck them into a tight ball. This causes the pump to cavitate/starve for oil.
Sometimes you have to drain the fluid to check the screen. Find the pump supply line and it should have some sort of bolt or screw on connection going into the tank.