I am having an incredibly annoying vibration/hum through my wall/ floor. It is Reverberating at about 36Hz. I am an audio engineer so I have the equipment to analyze it. It happens randomly throughout the day but mostly in the evening / night while I'm sleeping. It happens for exactly 30 minutes to the second. Sometimes it doesn't start again for many hours sometimes it will start again after a 10 minuet break but still always 30 minutes. I am in the 3rd floor of an apartment building. I have run around feeling pipes while it is happening to see what this could be. It's not the air conditioners. Sometimes it goes of 7 times a day. Sometimes it goes off 3 but always during the night. It sounds like a motor or compressor I can't sleep. I can't tell anyone when it's going to happen to call anyone to come out. I am going crazy. Please help!