SOLVED.
I contacted the support Motu (customer service) and they sent me a firmware updater in beta version (they ensure soon an official release version on Motu.com) aimed at solving these problems and disorders audio noises and troubles.
Here's how I eliminate problems, noises and interferences on the audio playback:
1) Formatted Mac Mini 2011 and completely reinstalled OSX 10.8.4;
2) Installed the official Motu Audio Setup for Mac OSX, downloaded from the website Motu.com;
3) Launch the firmware updater and updated the interface to the new firmware;
4) Power off and restarted the mac mini and the Motu Traveler Mk3.
After these operations I have not had any problems playing or recording, no hiss, noises, troubles or audio within suspicions.
Keep in mind that I have also powered the interface with the external power supply 12v, avoiding power via firewire, and selected external power switch on the side of the interface.
I also sent the outputs 1-2 (main out) of the Traveler Mk3 to a stereo input of a mixer (Behringer Xenyx 1832FX) and then, through connection with balanced XLR cables cannon, I sent the main out of the mixer to the monitors.
Probably this use of a balanced connection was eliminated noises and interferences sound.
I hope I might be able to help this update on the subject.
Health and Grace,
Good Music!
(See also this mine previous post describing my ex-problems:
http://www.motunation.com/forum/viewtop ... 73#p466173 )
Mac Pro 5.1(2010), Intel Xeon 2.8 Ghz, 12 Gb Ram, 1Tb Hd, OsX Mavericks 10.9.5; Motu Pcie424 with two HD192, Motu MTP Av Usb, Apogee BigBen, Soundcraft 600/500M Custom; Logic Pro 9.1.8, Logic X; MacMini (late 2009), Core 2 Duo 2.53 Ghz, 4 Gb Ram, 500 Gb Hd, OsX Snow Leopard 10.6.8.