Maybe something is getting inteference, make sure the motherboard is installed well with the standoffs in place and not touching the board (maybe you have one sitting there in the wrong spot), check power cables that they are not tangled up together, especially around the audio connection. Is the buzzing coming from the fans? Maybe they got knocked out of alignment when you moved the card to new case. Is the speaker buzzing coming from both front and rear ports and from headphones also?