The first thing you want to do is attempt to isolate it rather than keep guessing. Start with the preamp, disconnect the cables, if no hum, try the cables, try different cables, try unplugging turntable, in other words, make substitutions to try to isolate when and where the circumstance is that makes the hum. It sounds from what you have said so far that it could be anything from the preamp to the cartridge or anything in between.
A few ideas to try would be try mm inputs, different phono cables, unplugging leads from cartridge, unplugging nearby equipment.
A few ideas to try would be try mm inputs, different phono cables, unplugging leads from cartridge, unplugging nearby equipment.