Many people believe that the Hum does not actually get any louder at night but rather the way people hear the Hum
 changes as ambient noise dies down. As traffic slows and the world shuts down the noise around us diminishes and with less things to get our attention at some point our brains focus on a noise that was there all day but hidden among the many background noises. Certainly this is true but there is something else that occurs at night that most likely plays a role in why people hear the Hum at night.  

The Hum follows a pattern
For anyone who has been affected by the Hum for any length of time you may have noticed that not only does the Hum become more prevalent during night time hours, it may also conform to certain patterns. There are some exceptions but for the most part if you take note you might find that the Hum has a daily "routine" or a pattern that it follows. 

Many hum reports talk about after midnight and the early hours of the morning being the worst in terms of sleep disturbances and the feeling of being assaulted by low frequency sound waves. After many sleepless nights of researching how high pressure natural gas pipelines play into the Hum keeping me awake in central Alberta, this is what I discovered.

Gas days
In order to be able to account for how much gas is moved through a pipeline, gas transmission companies have what is know as a "Gas Day" A gas day is a 24 hour period with a start time that may be different depending on the region that you are in. This gas day allows the Gas Transmission Company to calculate and confirm how much natural gas has been moved through the system. In some areas the Gas Day runs from 8 am to 8 am. This actual start time may vary. A detailed explanation of a Gas Day can be found here What is a Gas Day ? and how it contributes to the Hum getting louder at night.
 

Line Packing
Gas can be temporarily stored in the pipeline system, through a process called line packing. This is done by packing more gas into the pipeline by increasing the pressure. During periods of high demand, greater quantities of gas can be withdrawn from the pipeline in the market area than is injected at the production area. This process is usually performed during off peak times to meet the next day’s peaking demands. This method provides a temporary short-term substitute for traditional underground storage. As " Line Packing" begins the pressure in the pipeline builds and the Compressors are forced to work harder and consequently generate more noise including low frequency noise during off peak hours. When demand rises at the downstream end as the world awakens the pressure drops and the Compressors don't have to work as hard. Consequently the noise diminishes as the pipeline unpacks.

Putting Line Packing in perspective
My investigation of the Hum has left me with absolutely no doubt that natural gas compressors and pipelines are at the heart of the matter here in central Alberta, Canada . Through a careful and open minded process of elimination I have also identified this as the most probable source of many of the Hum reports around the world. 

 Most people of the people who have been adversely effected seem to agree that the Hum is responsible for sleep disturbances and many reports include accounts of people actually going outside at night to look for the source of the Hum and being unable to find anything. Also common to most Hum descriptions is a low rumbling sound similar to a Truck idling outside.

Natural gas compressors and pipelines are quite capable of emitting low frequency sound waves that travel great distances. The sound waves are very long and often do not cause any disturbance until they reach a considerable distance from the source. Consequently it can be very difficult to pinpoint the source of low frequency noise.

Most people assume that the world becomes quieter at night as daytime activities cease and that is true. In the particular case of high pressure natural gas transmission via pipelines, Line Packing commonly occurs during off hours and periods when demand is low. That means at night and in the early hours of morning , Pipeline Compressors are working hard to pressurize natural gas and load pipelines to the maximum. When demand increases the next day there is a full supply. The result is an increase in low frequency noise pollution and in particular pulsations and a deep rumbling noise that for some people makes it impossible to sleep. This process of "Line Packing" is the reason that the Hum originating from high pressure natural gas pipelines becomes louder and more prevalent at night.