The amount of noise depends on what suburb you are in, sounds like you are under one of the Flight Lanes or near a Training Area. Most helicopter operations in and around Sydney fly at about 1000 feet unless they are within the Bankstown Control Zone, they do drop lower when they are flying along the the Lanes through Sydney Harbour and along the coast (500 ft from memory), also inbound from the west they drop height in steps on approach to Bankstown. Police and Medical Retrevial do have permission to fly lower when on operations but they mainly fly at the normal Class G heights.
Operations are controlled by the Sydney Basin Visual Pilot Guide (VPG) see:
http://www.casa.gov.au/pilots/downlo...NEY/sydney.pdf
for further info.
Noise also is dependant on the type of Aircraft as well, more modern AS-350, Long Rangers, Kawasaki, Bell 410, SkyCrane, have a very pronounced thump in certain flight conditions due to their blade design, others have a high pitched whine from the tail rotors.
Some things may have changed since I was in Helicopter Operations out of Bankstown a few years ago now.
Cheers
JohnG