I also had no idea the lady below me could hear it as well.
Recently I moved into a new apartment. I have been very happy living here too. Where I live is very nice and it is like a small neighborhood. However there has been one small problem with in my apartment. The weather has been warm so I have been using the air conditioning. Every so often the air conditioning makes a strange noise. Come to find out the lady below me had filed a complaint about the noise that she thought was actually in her apartment. When the maintenance guy came to check her apartment he noticed that it was actually coming from my apartment. So the noise was coming from my HVAC system but the lady below me could also hear it as well. So he looked at my HVAC system and realized that he needed to order me a new fan motor. The HVAC system was running and I could feel the cool air. However the fan that helps you blow out the cool air into my apartment was starting to go. He needed to order me a new one to replace the older one that was in the air conditioning system. So all along it was the fan motor making this little noise that I had thought nothing about. I also had no idea the lady below me could hear it as well. I guess I will have to start being more aware of the people who live around me. Luckily my air conditioning system will be fixed in no time. Now I hope nothing actually happens to the air conditioning.