Read paragraph 8 from the passage.

(8) . . ."I've often sailed in these seas, and I know that while some of the traders sailing between these islands and South America and other parts are decentish enough, others are [not]. But that's not the point. I have overheard, of late, some of the rascals plotting to murder the officers and take this ship. But I cannot point 'em out, for though I heard their voices I couldn't see their faces. I think I know who they are, but could not swear to 'em, and it would be worse than useless to denounce them till we have some evidence to go on. I therefore want you to help me with your advice and assistance, so that we may get up a counterplot to spoil their fun—for I'm quite certain that if we fail to—hark! what's that?"
This underlined sentence starts to build tension because