A Strange Sound

Last night as I was putting some things away in one of the spare bedrooms I heard a high pitched noise.  Wondering what it could be, I started exploring around the room but could find nothing.  I walked into the other spare bedroom and the noise was in there, too, but I couldn't place it.  Figuring that it would be easier to determine where the noise was coming from, I switched off the TV, which grabbed Casey's attention.
"Do you hear that?" I asked him.
"Yeah," he responded, and with a puzzled look on his face he began looking for this mysterious sound with me. 
"It's getting louder," I said.
"Yeah, I know..."
"What if it's a carbon monoxide detector!" I suggested, somewhat alarmed. Jeez, I wonder how long it will take for us to pass out? Should we call the fire department? Oh! It's a good thing this didn't happen during the day while we were at work! Sam and Guinness would have been dead when we got home!!!
As Casey walked into the other spare bedroom and I followed, he said, "It's louder in here..."
"The whole sound is getting louder!" I exclaimed.
He looked by the windows, and I decided to look under the bed.  Just before I got to my knees to peer underneath everything, Casey grabbed my Diet Coke bottle out of my hand and tightened the lid.  And the sound stopped. 
"Oh," was all I could say at first, "I didn't realize such a loud sound could come out of that small of a bottle!"
As usual, Casey just shook his head, laughed, and walked out of the bedroom...
Isn't it funny how we always - or maybe it's just me - automatically go to one of the worse scenarios you can come up with?