A little old lady goes to the doctor and says, "Doctor I have this problem with gas. They never smell and are always silent, so it really doesn't bother other people, but it's very uncomfortable for me. As a matter of fact I've farted at least 20 times since I've been here in your office. You didn't notice I was farting because they don't smell and are silent."
The doctor says, "I see. Take these pills and come back to see me next week."
The next week the lady returns. "Doctor", she says, "I don't know what the heck you gave me, but now my farts, although still silent, stink terribly."
"Good," the doctor said. "Now that we've cleared up your sinuses, let's work on your hearing."
[Many versions of this have been going around lately.]