TC reported unusual sound from below hiscab, unable to determine what kind of sound. Performance no t affected. ~~~~~ 3632 - TO REPLACE AIR DRYER.  3632 air dryer replaced & function test - OK. Air leak test 0.4 bar/5 min. #3632 Air Dryer S/N : OLD : 015-13537 NEW : 015-14348  Chan GF/Selvan/Alan 44216 3632 TC reported unusual sound from below his cab, unable to determine what kind of sound. Performance not affected.  3632 air dryer found frequent air purging & prolonged air discharge operation. Checked & found A2/B4 wheels having slight pitting all round. All othe U/F components & equipments checked - OK. OPS 20 raised to replace air dryer.  Chan GF/Hanafi 44216