Oh dear. I don't have a solution for you, just sympathy!

Live with it , lol ,
lets see what ideas you get here , but I think putting some thick rubber sheet under it might take away the humming .

If it's on your roof top deck, I hope you factored in some heavy duty support for it! Can you wrap it in insulation, the sheet kind they put into walls? See if it works, and if it does you'll need to cover it somehow, maybe with some yards of vinyl or have a handman make a facade for it.

Thanks! We definitely framed this section of the house with the weight load in mind adding extra joists and insulation in the ceiling...but the vibrations still carry. Hubby put some blue board insulation under the pump which helped but not enough. l like the suggestions and appreciate the ideas, thank you.

Sounds lame but adding potted plants to the area actually help absorb noise. Cushions from lawn type furniture close by also absorbs noise.

To ensure that this noise is "normal", read the info in the tag below. To me I just can't fathom that much noise would be generated from a proper installation, especially since extra insulation was added below the spa. You may actually have a different situation going on or a seal that has gone bad. Before you have total failure it might be best to call back a professional.

http://www.spadepot.com/spacyclopedia/pumps.htm

Maybe a sheet of sound absorbing dry wall under could reduce the vibrations and stop them from carrying