What could cause jerking and whining on a Caterpillar 330C with air bubbles and repeated return filter clog warnings?

[Gryks Machines]

Hey guys, looking for some advice on a Caterpillar 330C (2002, ~14k hours) I recently picked up. The previous owner mentioned a whining pump noise when warm, especially under multiple functions, and I'm seeing the same. Once it gets up to operating temp, it starts jerking/hesitating and makes a creaking sound under load.

What's also concerning:

• I'm seeing air bubbles in the sight glass
• The "return hydraulic filter plugged" warning keeps coming back

What I've already done:

• Replaced return, pilot, and case drain filters
• Cleaned tank screen and pilot screen
• Didn't find much metal, mostly rubber/silicone debris

After all that, the issue is still there.

 

[Miguel Flores]

Hey,
Sounds a lot like it’s sucking air somewhere, check the suction side (hoses, clamps, fittings, even the tank pickup tube) on that Caterpillar 330C excavator. Air + restriction can cause the whining, jerky movement, and keep tripping that return filter warning.