I recently purchased a Roland SC-55 MkII and a Yamaha MU128 and they sound great. I noticed that at some frequencies in the song I hear static. I checked the speaker connections and they look good. I am thinking the issue might be the contact between the RCA Outputs on the back of my hardware synths and the RCA cables. I do see some residue on the outer shell of the RCA Outputs. What would you guys recommend I clean them with?

-Tristan