Knell

Hear it not, Duncan, for it is a knell
That summons thee to heaven or to hell.

William Shakespeare, (The Tragedy of) Macbeth

Etymology

From Old English cnyllan (to toll a bell). Even though it's best known today from the phrase "death knell", the noun (the sound made by a bell) actually came from the verb.

The y sound in Old English mostly turned into a short i in Modern English (e.g. brycge -> bridge), but a few turned into an e sound, such as in knell. This is also why bury (Old English: byrgan) is pronounced (in most dialects) like berry. We spell it with a u because we took the pronunciation of one dialect but standardized the spelling based on another dialect.

kn-

In modern English, words that begin with kn- are pronounced with a silent k. We don't know exactly when this reduction happened, but most sources claim it took place somewhere between the 15th and 17th century, and it was a gradual process.

Many German cognates to English kn- words still retain the k sound (e.g. das Knie = knee). In most dialects of English though, the k is now only pronounced in proper nouns like Knoebels and Evel Knievel (and even then, it's less of a kn- cluster, and more of an extra syllable: kuh-noh-bel, kuh-nee-vel).

A similar reduction happened to words beginning with gn- (gnaw, gnome, gnarly, etc). If you've heard of a certain British song about gnus, be aware that in modern English, gnu "should" be pronounced "noo", not "guh-noo". (That being said, the word is what we make of it, and Oxford actually lists "guh-noo" as a valid pronunciation.)

Second of three bells?

There's a claim on the Internet that the death knell is the second of three bells announcing a person's death. No citation is usually provided, except for one Wikipedia article, which points to Henry Beauchamp Walter's Church Bells of England, published in 1912.

The book describes a Passing Bell, also called a Soul Bell, that was rung when a person's death was imminent. The book notes that "... in the eighteenth century it had fallen into general disuse and its place was taken by the Death Knell", which was meant to be rung at the precise moment of death, but practices varied. This was then followed by the Corpse Bell, also known as the Lych Bell, rung as the procession proceeded to the church.

The book does not claim these were the only bells rung surrounding a person's death, or that they formed a series of three bells. In fact, the book makes mention of at least two more bells: the Houseling Bell, which was a hand bell (i.e. not a church bell) rung by the priest as eucharist (bread and wine) was provided to the sick person; and an Obit Bell, rung on the anniversary of a person's death "for those who can afford it".

The death knell is the only bell referred in the book to as a "knell" though.