I have read that revelation sometimes used to come down to the Prophet, sallallaahu ‘alayhi wa sallam, like the ringing of a bell, and in another authentic Hadeeth, the angels do not accompany a caravan that has a dog or a bell. Please clarify this contradiction.
May Allaah reward you.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

Al-Haarith ibn Hishaam asked the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, "O Messenger of Allaah! How does revelation come to you?" The Messenger of Allaah, sallallaahu ‘alayhi wa sallam, replied: "Sometimes it comes down like the ringing of a bell, and this is the hardest of all (forms of revelation) for me. Then this state passes after I have grasped what he has said." [Al-Bukhari] It means that revelation comes down in a way similar to the ringing of a bell, it is not a real ringing.

Abu ‘Umar ibn Abdul-Barr said, "The ringing of a bell in the Hadeeth means revelation comes down and sounds like the ringing of a bell."

In this Hadeeth, the Prophet, sallallaahu ‘alayhi wa sallam, mentioned that the descent of the Angel on him inwardly sounds like the ringing of a bell.

A Hadeeth narrated on the authority of ‘Umar described what he used to hear near the Prophet, sallallaahu ‘alayhi wa sallam, "When revelation comes down to the Prophet, sallallaahu ‘alayhi wa sallam, we hear a buzzing sound like the buzzing of bees near him." [An-Nasaa’i]

Al-Haafith Ibn Hajar said, "‘Umar likened it (i.e. the sound of the descent of the revelation) to the sound of the buzzing of bees according to those who heard, and the Prophet, sallallaahu ‘alayhi wa sallam, likened it to the ringing of a bell according to his position."

Therefore, there is no contradiction between the Hadeeth and the saying of the Prophet, sallallaahu ‘alayhi wa sallam: "Angels do not accompany the travelers who have with them a dog or a bell." [Muslim] That is because a bell in the second Hadeeth is a real bell, whereas it is a simile in the first one. Imaam As-Suyooti, said,

"If it is said how is a praiseworthy matter likened to a dispraised thing? For the ringing of the bell, which is authentically proved to be forbidden and that the Angels do not accompany travelers who have with them a bell, the answer shall be: In similes it is not necessary that the two ends of the simile are identical in all characteristics; it is enough that they share one characteristic. The simile in the Hadeeth is to represent what is intangible by mentioning something the audiences are familiar with in order to be easily understood."

Allaah Knows best.