If you are using a solid state preamp there's possibly not much that can be done, if it is actually operating normally. Could compare the idle noise levels of each channel (turn it up with no music playing). If about the same then it's probably just inherent in the design, so then you just live with it or try a different model pre. If one side is significantly different then service could be in order.
A tube pre could explain higher idle noise levels caused by differences in tubes and /or degradation thereof. You may be familiar with "tube rush" noise often referred to here. Tube rolling, or swapping like devices between the two channels are some possible paths to disgnosis &/or resolution.