do not put WD40 I electronics period (wd40 is a cleaner with a mild lubricant not really good for anything imo). use a proper electronics cleaner. Also should note most volume pots are not very closely matched at their extremes ( fully off or fully on), you could have a bad volume pot too though.
it may also be you have to much gain in your phono section (CD) do you have gain adjustments so you can lower the phono stages gain? I know you said both cd and records so may be reaching there.