Ground loop hum is typically about 60Hz, a low throbbing hum, and indicates a ground loop problem. Higher frequency buzzing sounds like your normal run of the mill phono cartridge/phono stage noise. Two similar and related but yet different things. As you can tell.
What I think happened was you disconnect the phono stage this eliminates the ground loop and cartridge completely and all is silent. Connecting the phono stage alone introduces the ground loop. Then connecting the phono adds that buzzing noise.
Now with phono RCA and ground disconnected try the phono stage. If you get hum from the phono stage alone check where everything is plugged in. Try plugging the phono stage into another outlet. Try plugging the preamp into another outlet. If you still get hum like this the last thing to check is disconnect the preamp from everything except the phono stage. If there are no outlets you can plug these into that doesn't result in hum then we have probably ruled out everything and are left with a bad phono stage.
Otherwise, eventually, if the phono stage is okay then you are bound to find some way of connecting without hum.
Then connect the phono ground to the phono stage. If this adds hum, well let us hope it does not! Then connect the RCA. Hopefully this only adds the noise. Eliminating this is another similar process. Set that aside for now. Start with the ground loop hum.
