what does the phrase 'the low din' mean?

[Duc]

The low,unbroken......of the machine next door gradually bored its way into his brain.

a. din b. thud c. blare d. hum.

The answer is ''din'' but as far as I know, ''din'' is very loud noise and I think it can't go with ''low''. I guess the correct answer here is ''hum''. Am I right?
Please help me.
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[Jamie (K)]

"Hum" is the best answer, but sometimes it would be possible to say "a low din" with a somewhat ironic meaning.