Your customer will think you're a hero when you cut down the racket from their noisy garage door openers. In this video I hung the motor assembly from rubber pieces I cut from a tractor trailer's mud flap that I found on the side of the highway. (And you wonder why they say us New Englanders are resourceful.)
Hanging loud mechanical parts from rubber straps and bushings isolates sound very effectively.
There are kits on the market that basically do the same sort of thing I did in this video, but I think you should keep your eye out for a mud flap. Be sure to look both ways before crossing traffic.