Recently, I hear the very low noise when I stay home. Then I checked and checked it so many time but the noise is still on day or night and I couldn't find where the source of the noise coming from. It is not from the appliances at home but sounds like next door using
Dryer. Sometimes I walk through the alley, it is quiet outside.
Is there any thing will help me to check the source of the low noise? Thanks a lot!

Do you have tinnitus? (Ringing in the ears caused by being around loud noises.)
If so then this could be the cause of your problem.
Other Posibilitys;
Computer Hard-drive
DVR Hard-drive
Fan Buffeting
Radio on but low

It's hard to say. Hope this helps you out.