You have some air trapped in the housing. In most cases the air doesn't cause any problems. If you'd like to get it out, with the system running, unscrew the DI housing about a 1/4 tuen - just enough to break the seal at the oring. You'll see the water level start to climb immediately (have a towel handy). Just before the water level reaches the oring, cinch the housing tight.
Russ