[DMcMahon]

If the volume potentiometer (pot) is not a sealed pot and has any gaps around the 3 connection legs (or elsewhere on the pot body) then a spray of electrical contact cleaner will normally resolve crackling, unless the pot track is badly damaged/worn.

Also make sure that the heads are really clean using IPA/Methalated spirits and a cotton bud, water or spittle also will do a good job if the heads are not too dirty.

David