Grado/Rega Hum

The hum is loudest after the playing grooves.  When it is as feint as you say it is, no it's not compromising sound. When you're listening to the music, I bet you can't even hear the hum.

Fix the problem at the source. It is most likely due to a grounding or shielding issue. The RP3 being belt-driven is unlikely to be radiating enough EMI to bother the Grado, even though Grado is notorious for poor EMI rejection. First, determine when the hum goes away. If it is present only with the motor on, check the routing of motor power wiring to ensure it’s not parallel to the interconnect or tonearm wire at any point. If the hum is present with the interconnect detached from the tonearm, look at the phono preamp connection to the preamplifier. If the phono preamp power cord is not polarized, try reversing the plug polarity in the receptacle. If the hum is present with the phono preamp connected, but not powered on, try running a ground lead from your preamplifier to the phono preamp. Bottom line: you need to use process of elimination to narrow down the source of hum until you eliminate the cause. 

Sleepwalker,

 I have an external power source to the turntable (PSU).  The Rega has a non-detachable power cord.  Also, my preamp has a built-in phono component (Conrad-Johnson PV-11.)