siruis / noise help

hi i have a pioneer z1 and i am using the aux input to hook up to a siruis starmate. and i hardwired the power directly to the power going to the z1 and used the same ground. but when i turn it on i get alote of alternator wine and i can hear a faint kind of heart beat sound. strange im sure it is somekind of ground issue.. because if i plug the starmate into power from the house it sounds fine no noise .. also when i use power from the car the left side is very faint , but not when i plug it into the house power. and if i touch the center part of the rca on the outter part of the left rca in, it sounds fine. i have tried 2 different starmate recivers, and tried different power soruces in the car. same problem.. their is no other noise in the system(when i use cd or ipod or dvd) sounds fine. just when i use the car power with the siruis radio into the aux in.. anyone have any ideas.? i had it working before but then all of the sudden it was really noisy one day.. i tryed grounding the aux rcas to the back of the z1 helped alittle , but didnt change the fact that the left side is weak.. thanks alote for any help! its driveing me crazy..!