Are you hearing strange noises from a bathroom sink or from the washing machine drain in the laundry room?
The noise probably is the water in the p-trap of your condensate drain.
During the winter, the drain dries out and you are hearing the final dregs of water moving with the trash in the drain. Since the p-trap prevents sewer gas from getting into the air stream of your air handler, it needs to have fluid added back into it. Either vinegar or bleach will work to kill the stuff growing in the drain. Both work well, but in a pinch, you can add water into the drain to stop the noise.
During the spring, summer, and fall when the air conditioner is working and removing moisture from the air, it is a great idea to add bleach or vinegar monthly to the vent in the p trap to kill the stuff growing there.
Please note: if there is a vent or area to add bleach or vinegar between the p trap and the air handler, it must have a removable cap added so the air handler will drain properly.
Final note: If you have a tub or shower in the home not being used, often this time of year when it the air in the home is so dry, the p-trap for the sink, tub, or shower will dry out and a horrible stink smell will occur in the bathroom, just run some water in the drain to refill the trap and get rid of the smell.
Jonathan Smith has been in the Air Conditioning and Heating industry for well over 30 years and has spent more than 27 years in the Electrical industry.