i noticed that when pushing the bike along it made a faint sort of grinding noise from the rear wheel. well not really grinding but like the noise that you hear when you apply the rear brake but very lightly. so i sprayed some wd-40 along the disc and where the brake applies and it sorta went away but obviously braking afterwards was a bit shitty. i wiped it with a rag and kept braking hard and it's kinda going back to normal. any suggestions?