The sound is frowned upon and anyway: sniffing your nose, it all just goes into your sinuses! What children have been told for generations is not true at all.
Those with colds are in good company, especially in the autumn, winter and spring weeks: That is when viruses that attack the upper respiratory tract are at their peak. Cold viruses also attack the mucous membranes of the nose, which react to the viral invasion by producing more secretions. At first, the secretion is usually watery, later often yellowish to greenish, but in any case: annoying.