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Is it the original cartridge?
MM cartridges need a certain amount of capacitance for a flat freq response. The maker will typically spec the loading at 47K Ohms and some capacitance between 200-400 pF. The low capacitance cables like mentioned before really came into play in the '70s with the old CD-4 records with frequencies up to 50 kHz that needed 100 pF and 100K Ohms load to work right. With that CD-4 loading a typical stereo cartridge would usually sound bright.
Some turntables have a mute switch that operates while the arm is moving and landing on the record so you don't hear that initial noise of the stylus finding the groove (or lifting at the end). That could explain why you don't hear the hum until the record starts playing.
I assume there are no components with big transformers, or fluorescent lights nearby that could be inducing hum. And you have the separate ground wire and it's connected.
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