I'd start by checking whether the noise changes when the load on the pump changes. I experienced a similar humming noise after an oil change, and it later turned out that the problem was caused by air in the system. It’s also worth checking the condition of the suction line, the oil level, and the connections at the pump inlet. For one repair, I ordered Danfoss hydraulic components when I needed to replace several hydraulic parts. I also checked the filter and made sure there was no flow restriction at the inlet. If the pump starts making noise specifically after it warms up, I’d take a separate look at the oil condition and temperature. In some cases, the noise can be caused by wear on the pump’s internal parts. Does this sound occur immediately after startup, or only after it’s been running for a while?