Thrum

thrum
v. to play on a stringed instrument, as a guitar, by plucking the strings, esp. in an idle, monotonous or unskillful manner.
n. a thrumming sound.

[1592, from the noun (1553), of imitative origin.]

It's hard to not imitate the sound an acoustic guitar when you pronounce the word thrum, especially if you hold that emm. Onomatopoeia ("the formation of a word by imitation of a sound made by or associated with its referent") is fun. Bang! Boom! Poof!

A near-synonym for thrum, and more commonly in use, is the word strum ("to play on a stringed instrument by running the fingers lightly across the strings"). While strum doesn't qualify as onomatopoeia, it has an interesting origin: Strum was formed by combining string and thumb.

The guitar-word thrum has a homonym ("a word the same as another in sound and spelling but different in meaning"). The other thrum is "a fringe of warp threads left on the loom after the cloth has been removed." That thrum is not onomatopoetic. It showed up in English in the 1300s, about 200 years before the onomatopoeia was coined. It came from an Old English suffix -thrum (in tungethrum ligament of the tongue), and is related to Old High German trumme, meaning drum.