I have my setup on an 05 Xterra SE (RF premium audio - before) and I noticed that static/scratchy issue upon install. I decided to add some 2-more grounding points from the HU itself to the body and I think it solved that issue.
BTW, I had the mic mounted inside the inst. cluster shroud upside down (upper left if you're looking at it) I then give a super clear audio on the other line. I believe if the unit is getting enough/proper grounding your golden - at least I fixed my issue by adding more grounding. Try it!
Also check your mic wire/cable if it's really IN there at the back of the HU. Too much bending puts stress on the leads inside it and "could" affect the integrity of the audio it's transmitting and check if you got some "bare-ness" or cuts on the mic wire, it's where unwanted RF tend to interfere (IMHO).