If it's an attached garage set it up in the basement. And then pipe it up to garage. Cool air in the summer with low moisture and warm air in the winter. I used to have mine in the garage, in the summer it was a sprinkler. When I moved it to the basement the compressor ran cool and the moisture went away. And it is easy to service. Plus I don't have to listen to it, the wife gets to enjoy it! Pete
I'd mount it outside in a properly ventillated enclosure. Your attic may be strong enough to support the weight, but is it properly designed for the vibration?
a coople things i should mention, the garage has both radiant heat and air condition . i have no basement . the "attic" is more a mezzanine second story for storage of parts, etc. and will be climate controlled.. im hesitating though on putting it there, i think the dampening of vibrations may be a huge feat in itself...
I put mine under the stairwell leading upstairs, where the stairs turn. I am going to enclose it with plywood and thought about insulating it w/ noise type insulation if it gets to be too noisy.