The main causes of phlegm in the throat, without cough would include chronic inflammation, either allergy or infection, of the nose and/or sinuses with postnasal drip, chronic bronchitis usually due to smoking
cigarettes, asthma and acid reflux from
the stomach, often called gastroesophageal reflux disease (GERD). The seriousness of the mucus depends on the cause. You will have to work with your doctor to determine the cause and then get the proper treatment for that cause.