How noisy is the Mbox 1 Orig?!

[buckaroo]

I have a Mackie 1202VLZ3 mixer and Digidesign Mbox1

Now I have connected all my sound sources into the mackie as normal and the Control Room out to my monitors..

Now I want to record any combination of the outputs of the mixer into the Mbox1, so I have connected the MAIN OUTS of the Mackie to the Mbox ins ( as the Main outs and the Ctrl Room outs are mirrored so i can hear whats going on through the speakers)

I then want to listen to the recorded PC track from the computer back in through the desk and monitors, so have connected the mbox back into the TAPE IN's.

Now I can flip between recording and playing back BUT the TAPE INPUT (you can monitor this via the buttons above the CTRL ROOM/SUBMIX fader) seems to have a audible tone/hum on it?

Its not too loud but noticeable, but when i switch back to main mix the sound has gone?

I have done some re-routing and testing but have come to the fact that it maybe that the Mbox1 has noisy inputs/outputs?

Does anyone else have the same problem? - i prefer the Mbox1 as i have used it with other applications on the PC like Acid Pro and Soundforge via ASIO Drivers adn its been great and also it has its own INSERT points which they seemed to have removed on the mbox2!

Any help on getting rid of this noise would be great!