Firestudio mobile same issue. Using midi for Roland TD9. Spdif R/L alternate pop/clicking random. Changing clock source to spdif is no use as there is no external source - red/blue light flickers and no sound. Muting and zeroed out faders do nothing either.
CLicking stops if i remove the midi spdf plug - not useful for midi applications
Happens on both imac (lion 10.7.5, 3.5ghz, 32gb ram) and macbook pro (Sierra 10.12.6, 2.5ghz, 16gb ram)
Connected gear: TD9 on and off. Mandala USB.
Other than that the units hot