This is a very resonant word. I don’t mean that it sounds like a gong being struck, though it sort of does. I mean that it sounds like a number of other words:
Guam, a Pacific Island territory of the United States
guano, a word for accumulated bird poop, which came to English from Quechua via Spanish
gwan or gwaan, Jamaican patois for ‘go on’ meaning either ‘happen’ or ‘go away’; a popular phrase using it is dem fi gwan, ‘They should go away’, which you can hear in (among other things) Ruff Scott’s fun song “Tell Dem Fi Gwaan”
gun, though more by appearance than by sound
gown, which is what you get if you reverse the diphthong in guan
Guantánamo, which I do not need to explain, and Guantánamera, the name of a well-known Cuban song about a woman from Guantánamo
Juan, especially if you’re really harsh with the j
guan, which is more than one word in Mandarin; it is two different surnames (depending on the tone), sometimes transliterated Kwan or Quon, it is a word for ‘shut’ and ‘barrier’ and a different word for ‘pipe’ – from the latter it names a double-reed woodwind, a sort of Chinese oboe (not that it sounds all that much like an oboe)
It also has that fish-like oral gesture that sucks air into the mouth, as you may see in quantity or French quoi.
What actually does guan signify? It’s the name of a South American bird roughly the size and appearance of a turkey – or perhaps a bit like a chicken with longer neck and tail and different colouring (typically black). Although the name could be a sound a bird would make, it’s not the sound the guan makes. English got the name from Spanish, which probably got it from a South American indigenous language (though it’s not certain which one).
There are actually several kinds of guan, some with brighter feathers, some with a big crest on the head. Here’s a video of a guan: http://www.youtube.com/watch?v=Wt18fK44myA
You may wonder whether it deserves such a resonant name. But for all we know, it thinks the same about you. Anyway, something has to have the name. Might as well be this bird.