audio hum

hi
I have noticed a hum on my phono set up.
it is Not on my cd inputs.
I use harmonic tech magic 1 and 2 interconnects-all are
balanced except between turntable and phono preamp.

I have 2 dedicated power lines.
I use a vpi super scoutmaster turntable with whest
MC REF V dual mono-2 channel phono preamp plugged into
a BAT REX preamp to 2 bryston 28SST2 monoblocks.

I notice I very slight hum that starts slightly about 2/3 of volume on the preamp that is NOT there on the CD input or other inputs.

I do not notice at lower volumes and only can here it when vinyl is not playing at over 2/3 volume and it does NOT at all interference with listening-it is just here. When
vinyl plays, I can't even tell that it's there.

if I disconnect the interconnect and ground between the turntable and the dual mono phono preamp, it is still there.
I do have a grounding cable between the vpi and both of the
whest phono preamps.
any thoughts??

I have tried another harmonic tech Magic interconnects with similar results. It initially was that I could move my interconnect HIGH and it would diminish greatly. Then the interconnect was lowered it would appear again.
I moved the BAT REX power supply under the cd player, and NOT under the phono preamp, where it originally was, and it may be less noticeable, but is still there.
any thoughts are helpful.
could this be a ground loop between 2 dedicated lines?
but I thought that would be heard all the time, and not only when the volume is increased?
thanks