The low frequency sound has been a real bear to fix.  My exhaust sound is hardly anything.  It goes through eight feet of insulated exhaust pipe, then through a gas water heater to reclaim the heat, then through a car muffler and finally outside.  But the very low frequency sound, ie vibration from my six horse lister was amazing.  It rattled my dishes.  It rattled my neighbor's dishes too.  That was with a rubber isolated frame under the motor/generator.  I finally chopped a 2'x 4' hole in the cement slab and poured an independent slab that's one foot thick and surrounded on all sides by 1" white styrofoam.  THAT pretty much fixed it for the neighbors, although you can still feel it in my house.

More fun than a barrel of monkeys...

And your tractor odors are while you are running biodiesel?

Best regards,

First, the engine itself can be enclosed and sound proofed using the same material used on pleasure boat engine compartments. It is relatively thin, heat resistent, and done properly dramatically reduces transmitted noise. It's readily available from most chandleries and not too costly. If it's air-cooled then it's a problem because you've got to have vents or ducts to provide cooling. That's my problem, but I'm in a marina that operates diesel engines all day so no sweat.

Second, the garage or shed needs to be modified to serve as a "disconnect" to the noise. This is done with a sound absorbing barrier that cannot have any holes or gaps in it and, ideally, no connections that can serve as sound conducters across the barrier. The best material for this is one that is often used in studios. It is incredibly dense. It is only about 1/16th inch thick, heavy (1-2 lbs per square foot), and resembles a blend of lead and rubber. It is by far the most effective sound deadener money can buy. It should be hung loosely, if possible, between partitions and form a complete seal. It works by absorbing the acoustic energy rather than reflecting it back.

Your engine and generator set needs to be installed on an isolated platform to prevent conducting the sound through the floor and outside the confinement. Steal a secret from the Navy's submarine fleet and use floating decks to isolate the equipment. These are floors that rest on rubber mounts. Should be able to rig a simple imitation by slicing up a tire with an angle cutter so that you have a number of U-shaped sections. Bolt the rim side down to your floor and the tread side to your platform. Use enough to support all the weight, which could be quite a few. Whatever solution you choose, avoid using fasteners that will simply conduct acoustic energy around the dampers.