Rattling in my ductwork

I almost never lay quietly in the condo office. Most of the time I am either playing a game on the iPhone, playing on the tablet, or playing on some application. I can hardly think, when there is no noise around me, and yesterday, I was sitting around in the office by myself, but the a/c system wasn’t functioning. The whole room was completely quiet. That’s when I heard a faint ding sound in the air vent. There is an intake air vent located right next to our tablet, however on a bright and sunny afternoon, I can almost see inside of it! When our office was quiet, I heard the strangest thing come from inside the air vents. At first, I thought I was going crazy. It sounded like someone was trying to communicate in a type of Morse code. I listen for many hours, as the noise continued to occur. I strolled around the house, carefully looking into  all of the additional intake air vents. I did not hear any other types of sounds in the house, and the noises appeared to come from the single air vent in our office. I eventually decided to leave the office, until our hubby came condo later. I described the horrible sound. My husband sited his ear next to the intake vent, however he did not hear anything. The people I was with and I turned off all of the electronics in the room, until the whole locale was one hundred percent quiet. My hubby later placed his ear to the air vent, and he finally heard the faint noise, he told me that was the sound of our oil furnace breaking. I cannot suppose that I never noticed it before. I’m ecstatic we did not have anything spooky where I am living.

air purification