The feeling of emptiness that Mr. Nilson has just under his fifth rib could well be the result of a psychosomatic condition. That is to say that he has a physical ailment which has been caused or aggravated by mental factors, such as stress. We know that Mr. Nilson has a high-profile job in the city of London, Great Britain's financial center and the British equivalent of Wall Street. And it seems safe to assume that his job is a very stressful one indeed. That being the case, it's not unreasonable to conclude that the strange empty feeling beneath his fifth rib—in the vicinity of his heart—is somehow related to his work.
As for the "sweetish sensation" in the back of his throat, that would appear to have been caused by the budding bushes of spring. It says a lot about Nilson that he seems to be so disturbed by such a sensation, which most people would welcome. It's only when Nilson finally tracks down the source of this sweetish sensation—the eponymous Japanese quince—that he's able to respond positively to the beauties of nature on this fine spring morning.
But even so, Mr. Nilson is unable to appreciate the beauty of the Japanese quince for very long, as his moment is spoiled by the presence of his neighbor Mr. Tandram. Tandram reminds Nilson so much of himself, and because of this, he feels painfully self-conscious in the other man's presence, making it impossible for him to enjoy this beautiful shrub for more than a brief period.