I am working in a large office building with a lot of people. Pretty frequently there is a very weak and low frequency noise that is very irritating. Unfortunately, I have found just one other person that can perceive it, so it must be at the utmost borders of what a human can hear. I have started searching for equipment that can detect the origin of this sound, but after some hours worth of effort I have not come up with much except some postings on this site about array microphones.

My questions are:

 - are there solutions to this problem out there (that do not cost 1000s of $)?
 - are there suitable array microphones that can (ideally) be plugged in via USB that can detect such noise (the cheaper the better)?
 - which type of microphone can detect weak, low-frequency sounds? Can those microphones be connected using USB?
 - are there other ways to locate the source of the sound? Maybe I'm overlooking ways to solve the problem.