The only time I typically notice the X15 is when it does it's idle seeks to keep the heads cool. The repeated "ticks" used to bother me, but you get used to it. It's a small price to pay for performance. It does have a gravelly noise to it during heavy access, but the system fans tend to drown it out a bit.
Get the Atlas 10KIII if you want a less noisy drive with great performance.