The noise is probably not at the 17hz which is what you need for the machine to do it's job...it's maybe setting up some resonance in something else which can be heard.

I don't think you will have much luck quietening it. If you really want to i'd suggest getting a mic and audio spectrum analyser to try and work out exactly what frequency is the main problem and exactly where it's coming from.

(I reserve the right to be wrong)

Del

PC, Mike and free software will do this.

Low frequencies are not well absorbed by glass wool.

From the field;

Are you certain that you don't have a torn or cracked weld? I work on a lot of smaller "swako" shaker machines and the are usually pretty stable until something cracks or comes loose.

Let us know.

Mike

Synchronize the balacing of both motors.Check motors for Phase Unbalance and also check mechanical arrangement functioning parts for any hinderence in performing motion and check its balancing also.If the system itself is having problem then there is no use of putting vibration pads or any other arrangement.

AS Del mentioned this is probably a resonance.

If you can find the panel or other object that seems to be vibrating almost as much or more than the motor and add mass ie weld heavier plates or i beams to stiffen it may alleviate some of the sound or change the frequency.

If you can lower the frequency to the next down harmonic 9Hz or so it will be much less annoying to the workforce.

This is a lot of energy so it will take large measures to make any difference.

Also I am unfamiliar with the operation of the machinery, and this may destroy it if not properly thought out (engineering) but could you reverse phase of operation on one of the motors , this would cause a sonic cancellation.

Pleasse consider carefully though as it might cause equipment cancellation if the machine would be forcing against itself or rocking and bucking in doing so.