My baby is 4 months old and wakes me up at night?

When my son was 2-3 months old he was able to sleep through the night and only wake up once around 4 to eat. Two weeks ago he got sick and now he is doing better but he wakes me up randomly throughtout the night to put his pacifier back in his mouth. I really would like to get him back the way he was so what can I do to help his sleep through the night? I have heard of a white noise machine that can help babies sleep through the night, any options or does anyone know if this will work? Help!