yep, air in the lines.
I had this issue too, and eventually verified the shower lines hold air for a long while in my 31L. If you ever let your pump pull from an empty tank, it'll happen again.
I found that sometimes it takes a long time of letting the water run to let it work itself out. Once the air is gone, the pump will run a lot quieter.
If you are at a full hookup site, just take a really long shower....or two....
Otherwise, it will eventually clear up