Hum in Garage Door Opener

About a month ago I installed a ¾ hp Sears Craftsman chain drive garage door opener to replace an older unit that gave out. The opener works fine, but yesterday I noticed a rather loud constant humming noise coming from the opener unit. I have tried disconnecting the door to make sure there was no strain on the unit but that did not help. I tried unplugging the opener and plugging it bank in but that also did not help. If the opener has been unplugged for a while, the humming noise does not start immediately when it is plugged in again. It generally takes 10 to 15 minutes as if something has to warm up. I have had several garage door openers over the years but have never had one make a noise like this. Any thoughts?