It's really common for digital servos to be quite noisy. The 615's are great servos, and the 655 is amazing! I have that servo on my tail on my 600 and it's solid as a rock.
If you want to upgrade servos though, look at Savox for about the same reliability, or MKS for top of the line. There are also some BK servos (Bert Kammerer designed them, .03 second speed, I"m talking smoking fast) but you are going to have to put a second mortgage on your house to get them.