squeaking planet x model c's

tubbs_214

right over the past few days ive noticed my rear wheel has developed a squeak while rolling both under pedaling and free wheeling.

i know the bearings are sealed cartridge bearing but as the wheel is spinning it still feels smooth could it just be abit of crap on the outer seal of the bearing and if i was to take the cone off and clean them and get rid of the salt( the problem started after a ride on some just gritted roads) would that sort it

ive service my old wheels a few times but there open ball bearings and as its my first set of sealed bearing wheels i didnt want to just open them up before i got some advice

thanks alot for any help

mark