+1

Cable friction is very likely the main culprit. Learn to adjust derailer systems if you can't already.

But remember, the very best habit you can learn with derailers, is whenever you shift against the resistance of the derailer spring, go a little past, hold it for a moment till shifting is complete.

A quality 9sp rear system will likely do the job of clean quiet shifting for you. I go with XT gear these days - best bang for buck. A hub is great but a much bigger expense.

Also, get a 9sp chain from KMC with X-plate profile. They are really very good indeed. It will work well with the 8sp system.