Try this, find a clip on the tape that has the noise and capture it, then capture the same clip with a different name do this untill you have 4 copies of the same clip. See if the noise is in the same location (use the audio waveform setting in the time line). If the location is different then you have a problem with 1-Cables (Firewire and audio), 2-Power (if you are on the same circut as the refridgerator it will create noise) or 3 – your FCP Preferences have gone gonzo. I have seen this problem fixed by trashing the preferences It is quick and easy so I would try that first. Then I would try changing out the cables. The power situation could be tricky but if you are not using a good UPS-Filter for the system that would be a good place to start.
Best of luck
Drew
Chaos is the beginning of everything.