Ghosts in my Ultralite Mk3. The mysterious hum chronicles.

[tangucho]

Hello. First of all I would like to say that I have no idea what is going on with my Ultralite Mk3. Actually I solved the problem but I am afraid it comes back as it went away recently.

My problem is hum. Mostly like a ground loop problem.

My setting is:
iMac 3.06 Core 2 Duo 4GB RAM 1TB HD 7200rpm
Motu ultralite MkIII Firewire
Samsom Studio Dock USB Monitors

I had a horrible HUM when I plug my guitar to Analog input mic 1 and 2. But when I use analog 3 I get a clean but ultro-low signal. I plug and re-check any cable and connection.
Finally I reset my MOTU to factory settings....same HUM.

Suddenly when I was switching channels to make some final tests...voila mic 1 and mic 2 and hum free.

Any clue of what is going on here?