I was unable to find much info when searching for drive line noises, but I searched for N55 noises and came across a few people referencing the steering pump. I noticed my noise got louder the closer the rack was to a full lock, left or right, and much louder at full lock. I flushed out the old fluid with CHF 11S and the truck is considerably quieter. The fluid was filthy black, it was probably never changed.