You can observe 2 more days to see if your bb has "ng ng". It's normal for BB to have "ng ng" once a day or once every several days. You may try to give BB some water so BB can "ng ng" more easily. If more than 3 days no "ng ng", then should go to see doctor to figure out what problem.
My BB is about 6 weeks old and she did not have "ng ng" for about 6 days. I brought her to doctor and doctor said she has constipation. It's because BB cannot force the "ng ng" out. The doctor then gave me a medicne call "Sur-lax" to 通便.
It's important to make sure that it's not sickness which cause BB no "ng ng".