Distorted sound from Focusrite 18i20 with iMac 27"

Hey guys,

PLEAASSSEEEEE HELP ME :(,

My focus rite preamp was working fine up until i bought an iMac 27inch. When i plug it in and start protools to record..i get a distorted sound even when the gain is low. I tried the same just with my monitoring station and the preamp without the iMac it all worked fine.. I need a solution PLEASE IM DESPERATE as iv got clients waiting and this is just so sad and frustrating. the distortion sound only happens when one speaks into the mic .

PLEAASE PLEASE HELP

Thanks

Regards

Col.