In one case, this problem was a ground-loop hum caused by running the Firewire410 on Firewire bus power with the Mac laptop plugged in to AC power. We found that unplugging the laptop (using battery power) eliminated the noise.

We also found that plugging the Firewire410 into a 4-pin Firewire port (non-powered) and using the Firewire410's AC adapter eliminated the noise (with the laptop plugged in to power).

NOTE: DO NOT plug in the Firewire410's AC adapter if it is plugged into a 6-pin Firewire port (bus powered)!! This can damage the Firewire410. The only exception to this rule is 6-pin Firewire ports on PCMCIA cards, as the PCMCIA bus doesn't provide enough power to run a firewire audio interface. If you are using a PCMCIA firewire card, DO use the Firewire410's AC adapter.