Cover the mic and see if it still occurs, if it does then the mic is picking up the interference via the cable. Did you fit the new card in the same slot as the old one? If not try that. It may be that the fan is of a different type to the old one. If it is controllable then it could be the coils of the fan causing the problem in which case its a screening problem. Other things to try are moving the mic cable as far away from the fan as possible.
Hope this helps.