بسم الله الرحمن الرحيم
(Fatwa: 349/296/D=4/1439)
While reciting the Holy Quran if you keep something inside your mouth then you cannot read the words with correct articulation. You may pronounce the word incorrectly. Hence it is against the etiquette and makrooh to recite the Holy Quran while having something in the mouth. In case the thing which is kept in the mouth it is pan or gutkha etc then sometimes it carries bad smell or stored saliva which is more heinous. The etiquette of reciting the Quran is that the mouth is cleaned completely so that neither there is any kind of bad smell nor anything else in the mouth.
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband