||04-29-2013 09:50 AM
There is a coupler between the motor unit and the screw. Fairly common for these to break, which in turn allows the motor to run but the screw does not turn. Not there to hear how your opener sounds so I can't say for sure there is not a larger problem. Of course the motor will sound quite a bit different without the screw turning. Without seeing a visual confirmation that something is turning they do often sound like they are just vibrating and buzzing. The motor probably is running inside the cover, you just can't see it.
The coupler is pretty cheap and an easy fix. If you do replace it I would suggest something other than a screw drive. A Chamberlain belt drive is a very smooth quiet opener, chain drive is pretty good also and a bit cheaper. Screw drive is way overrated and overpriced.