Coughing only sickness

Hi mummies, my 2 year old son recently started coughing. Sounds like there is phlegm when he cough. It's been around 4 days already, but he doesn't have runny nose, just coughing sometimes during the day and it will worsen at night. Is this normal? Cause usually if he caught a cold there will be runny nose right? But he doesnt have runny nose or other symptoms. Just cough only and there is phelgm base on the sound. What could be the reason for this cough? Would appreciate if any mummies can help advise! Thanks!