Here is our situation.
Brand new home closed end of June 2005. House located in Florida. 1809 square feet with detached rear garage. There is also an apartment over the garage. House is 52 feet wide. Brick construction with steel stud interior framing.

Our problem is when the outside a/c unit is on a low frequency humming like bass sound travels through the left side of the house. It is unbearable when it is on and impossible to be in the rooms affected.

Unfortunately it is mainly in our master bedroom/bath area.

The unit was originally located right outside the master bedroom. The builder and their sub (both major names in Florida) have sent out 8 different techs and their work has not solved the noise problem

They first installed compression pads under the unit to attempt to take away vibration. That did not work.

Then they moved the unit from the side of the house directly outside the master to about 35 feet away next to the rear garage. The noise is still present and did not lessen a bit - and now is heard in the apartment above the garage.

As far as construction, the a/c lines travel underneath the foundation in a chase from the width of the house starting at the master bedroom to the laundry room where the interior unit is located.

At one point they thought the lines in the chase were causing the vibration. Keep in mind the lines travel the width of the house and then another 35 feet in the yard to the location at the back unit.

When the a/c powers on it sounds like heavy machinery is starting and then the humming noise permeates the entire left side of the house.

Here are the weird things - 1. The unit is now next to the garage. If you look outside from the gathering room which is next to the master the unit is right in the middle - but there is no humming bass sound in the gathering room only on the side of the house opposite the inside unit.

Also you can now hear the same noise in the apartment above where the unit is located.

The final straw concerns the neighbors unit. During the analysis, all units were powered off in our house. The neighbors unit was also powered off. We they turned on the neighbors unit the bass humm was in our house, and then when we turned on ours it just added to the sound.

You can't hear the noise if you are in the attic and you can't hear it if you are outside. Sometimes it travels in waves but mostly constant.

The builder now wants either place some type of blanket over the compressor or to replace the unit/compressor.

Any thoughts? Anyone ever experience anything like this - I am hearing the nosie as I type.

Sorry for the long post - If you made it to the end thank you for reading. We have been sleeping in the dining room - the room on the very opposite of the house. There is no quiet in the house at all.

PS the unit also never shuts off - it is constantly turning off and on. I suspect it may be a result of the thermostat being installed on an outside wall?! Our first power bill was $309.00 - very high compared to the 4 other houses we have owned in florida.

The only time I have encountered this type of noise was when the motor or squirll cage was out of balance. That created a low rumble vibration. Could be both units are the same type or installed by the same incompetant tradesman.

The only times I have run into this is if debris is lodged in the blower fan or fan/motor is out of balance (but that's usually heard through the house), OR (and this may be the most likely problem) if the refrigerant lines are touching some type of wood framing where they pass through the exterior wall or floor joist or inside a sleeve and vibration is being transmitted through the house, possibly next to an area serving as an echo chamber. At least four times in the last year we quieted vibration (humming) in different houses by one or two well placed shims or spacers around refrigerant lines, or bending copper lines slightly.

I wonder if the large exterior fan is moving air in a wave-like pattern, causing some type of sound wave vibration. Try a different fan blade. Two homes with the same hum could be design of exterior fans echoing between houses or off the exterior walls?

It's either that or your neighbors music is too loud.