Question
We have a 7-zone sprinkler system (Rainbird) that was installed four years ago. Recently, we've noticed that when the system is running, a "grinding/humming" noise can be heard in the pipes inside and outside of the house.

The sound can only be heard when the sprinkler is operating in certain zones. It's usually the same zones, specifically zones 3, 4, 5 and 7.

The sound goes away if I turn on the bib (faucet) outside the house, leading me to think it may be a pressure issue.

Any help solving this would be appreciated!

Answer
You are right about the pressure.  Sometimes you can reduce the noise by adjusting the flow control for the valve.  It most likely needs to be open more, but try different adjustments if one doesnt work.  Also take a look at the link below, there is a good explanation of the condition of water hammer, and some solutions.

http://www.irrigationtutorials.com/waterhammer.htm