I have reviewed the other questions in regards to the humming sound, but have not been able to apply them to my home situation.

I live in a condo with 3 units (side by side), one floor. At about 3:00 am in the morning I hear this "humming" type of sound, and sometimes during the day a "light tinny sound" (like a small fan is out of whack).

I do not have any electrical or water appliances which are set on timers. This started about 3 months or so ago.

I have turned of all electrical, item by item and as to the MAIN SWITCH and the sound still comes around. Actually with the time a change a month or so ago, the sound started on hour later (daylight savings time).

Its one of the sounds that at 3 am can sound louder to me since I live in the house and used to the sounds. The air conditioning people were here but did not hear anything or find anything wrong with the a/c (its only about a year old - with the AC I have an evergreen clean non-filter, which uses ultraviolet light to convert the dust etc. into water).

The only work in the last few months has been to have the laundry vents cleaned, and the 'outside flap" one of the vents for the toilets closed (It had been opened).

Also had to have the duct pipe to the above toilet vent hooked up correctly.

The neighbor (adjoining) has a water softener which apparently needs servicing. I have spoken with them and they turned to water softener to bypass - but the sound (and we believe it may be from their water softener) still comes on at about 3:00 am. It generally disappears about 10 am.

Thanks and hope someone has a suggestion. I am also going to call the local water department this week,

Thanks,

Turning a softner to by pass does not stop the regeneration cycle. Ask him to unplug the unit and I bet you noise stops.

Thanks to ballengerb1. Neighbors are stopping over in the next couple days. I believe they have unplugged the unit but will pass on the suggestion. I was also told even if you have pulled the plug if it is in the middle of the cycle, the cycle will continue,

Pulling the plug stops the cycle since all solenoid valves will close and return to their at rest state.