This was reported in the New Zealand Herald back in 2006.
As far as I know there never was concrete answer provided to answer peoples questions as to what is makeing this noise.
Mysterious humming driving Aucklanders 'bonkers'
12:00AM Friday October 27, 2006
By Kate Chapman
A mysterious humming driving people to despair across Auckland has pricked the ears, and curiosity, of scientists trying to find the source.
Massey University computer engineering scientists Tom Moir and Fakhrul Alam have been contacted by more than 30 people, most in Auckland and the North Shore, who claim to have heard a humming noise.
The symptoms are similar to those suffered by people with tinnitus, commonly associated with a prolonged high-frequency ringing in the ear.
The scientists are visiting people who can hear the humming, and trying to measure the noise in the hope they will be able to identify its source.
People from central Auckland to Kaiwaka in Northland have reported the sound, as have residents from the North Shore suburbs of Torbay, Browns Bay, Murrays Bay, Birkenhead and Beach Haven, and Stanmore Bay on Whangaparaoa.
The scientists are measuring the frequency by playing a second low frequency to someone who can hear the humming. When the person can no longer hear the hum, the frequency they are playing is the same as the humming noise.
Dr Moir said people could definitely hear a sound. "It's quite serious to them, it's driving them bonkers. I was there at the same time and I couldn't hear anything."
He said the sound was well within the average hearing range for people.
Most people can hear between 20 hertz and 20 kilohertz and the humming is around 56 hertz, according to Dr Moir's research.
Not everyone could hear the sound, because of its low volume, he said.
"We're all born differently - some people are better runners, some people are better hearers."
[link to www.nzherald.co.nz