A low hum in Patterson Lakes, Victoria 3197 Australia

Low rumbling Yes, I think have something to add to the subject I have/had experienced three hum/rumbles. The first one I believe is a “memorised” noise, I was awoken one morning by a large diesel truck idling outside our bedroom window, after about twenty minutes I told my wife I had had enough and I was going to give the driver a piece of my mind. My wife looked at me strangely and asked what I was talking about, I said can’t you hear that damn truck? You must be deaf! I was in somewhat disbelief to discover there actually no truck parked outside my property, and the sound soon disappeared. This is a very occasional occurrence. The second hum is as described by most other people, a low rumbling sound, I have actually confirmed over the years that this is entirely in my head. I fact I know what brings it on fatigue and tiredness is the culprit. Before I retired I used to be self employed and would often work late into the night, once I was tired, and with no audible sounds around in the house, I would start getting a low rumbling slightly vibrating sound in my head. I believe this might have been made even worse by some stress on top. Listening to music or voices via headphones, or sleep was the only cure, however it would often continue the next day, and playing distracting music was the only way to get rid of it. Now I am retired (76) I rarely experience this sensation any more. My third is a “Rumble” which took me a long time to figure out, but in the end was so simple! Our home has ducted heating and cooling, it is actually a commercial sized split system. The is consists of a large sheet metal “distributor” box above the lounge room, with flexible ducts to every room, and a large sheet metal “collector” box at the end of the hallway with two flexible return ducts to the distributor. I have an office in the spare room with a duct outlet directly over my head, I finally worked out that the distant very low rumbling sound I could hear (only on certain occasions was coming from the vent. The sound appeared to be coming from underground, was only apparent when the aircon was no running, and when the rest of the house was pretty silent. My wife could not apparently hear it, which was surprising since my hearing is starting to get a bit average and hers is good. What I believe I tracked it down to was temperature changes distorting the dirty big hanging sheet metal distributor box, with the sound travelling virtually the full length of the house to the duct in my office. I am certain that is the source of the sound, but I cannot be so certain of the cause, just an educated guess. I hope this info is of use to somebody experiencing similar issue.