the drains to sump- are they submerged or hanging above the water? if it's the latter, there's your problem. submerge them and it will get quiet.
i don't know where you live, but here, in my garage, it gets below freezing in the winter. i would suggest against having the sump out there. it would be a tremendous battle heating that water, especially at night when the lights are off. the temp swings will be immense.