How many tracks have you recorded? Do you hear the noise in all tracks? Is it momentary or permanent? Does it happen at loud passages of your recording...?
Recording takes place right after the preamps without any processing of the board, unless you activate the "Post" button (Digital Output) on the board's channels. If you have not recorded the processing (EQ and Dynamics), the problem probably will be found on the input side (microphone, DI-box, cable, preamp, insert).
There can also be a problem with firewire (cable, connectors/connections, card). Check your cables and settings and do some test recording. This should give you a more clear picture. And last but not least check the sampling rate of the board and Capture. They should be identical.
