I have exactly the same issue, 2-way sli, crackling only when in sli, however there are THREE situations when this occurs for me - please check if you have the same:
/ mainly when in games viewing bright colours (e.g. sun in FarCry2) - as already confirmed by you
2/ when Raid0 is being used which also uses the bus, e.g. FarCry2 during the load screens. Do you have Raid0 to reproduce this?