If you're muting with the other hand (ie. the hand you have on the fretboard), and you're still getting a ringing noise when you play the chord, you might find that you are inadvertently depressing one of the higher strings with the base of your finger (I usually have this problem when I'm playing barre chords in quick succession). Make sure your fingers are as straight as possible against the strings, then try strumming the muted chord, progressively adjusting the pressure on the strings until you attain the ideal muted sound.