Its one of the isomers of nonenal, formally speaking, the conjugated unsaturated aldehyde, non-2-ene-1-al that stems from a 9-carbon straight chain hydrocarbon. Although I doubt very much it is produced in-vivo from nonane. Not dissimilar entirely from the likes of fatty acids that make rancid feet or sweat smell, well, rancid. Or that give vomit its distinctively puke-like stink, like propanoic, isovaleric/valeric acids and in the case of barf, butyric acid. I've never worked with pure non-2-ene-1-al, but propionic acid as a side product I've experienced and its so much nastier than acetic acid as from vinegar, GAA etc. And butyric acid, which gives throwup its nasty smell, I once accidentally had some n-butyric anhydride get into the oil in my vacuum pump whilst distilling off unreacted anhydride from something and it STANK the place up something terrible, belching a stinking miasma from the air vents that smelled exactly like vomit. Oh and once had to remove it from something by means of a homespun negative pressure chamber, and during pumping it down it smelled absolutely filthy, and so did the chamber when I was finished. Stank like a city full of already pretty rancid football fans had used the place as somewhere to stagger to in order to empty their stomachs of really cheap and nasty super strength lager.
And it doesn't take much either, one drop of butyric anhydride, although it doesn't hydrolyze completely, immediately, is instantly enough in a large room to make the entire space smell like the inside of somebody's stomach complete with a semidigested last meal. And I'd imagine that if it were the acid rather than the acid anhydride, it'd smell even worse even faster, although I've never handled neat butyric acid (nor, for that matter do I ever want to, since that is pure, distilled, undiluted essence-of-sick.)