Your Macbook's power isn't "clean" because of the electronics inside. The hight pitched stuff when it runs on battery is actually the processor switching clock speeds. It does that quite often (few hundred times per second) to save power and run cool, you can actually hear it if you press your ear against it. It's more pronounced when the computer is idling. Move windows around and it quiets down. Most electronic devices have that to some degree, calculators, flash based iPods, cell phones, etc. Actually, running photobooth and just hiding it makes it stop because the CPU is kept from switching speeds for some reason.
This electro-magnetic disturbance is passed along through the Firewire cable. and also through the power lines in the house. You can improve it a lot by just using a different power outlet for the computer than you do for the audio interface. 6-pin firewire cables (the larger ones found on audio interfaces) all transmit power, the 4-pin versions (used for DV cameras) don't. So you can eliminate the "dirty" power through firewire cable by using a 6-pin to 4-pin cable and an adapter to get it to 6-pin again, eliminating the power transmission through the firewire cable.
The noise you hear when the Mac's power brick is connected is the power controller checking the power status and/or charging the battery in short bursts.