We own a Tenpro Prolift Professional Model 200 Garage Door Opener (chain drive) for our sectional garage door

The problem began when we noticed the garage door not closing all the way; It would open fine but on closing it will come to a stop about 2 inches from the ground. This wasnt a problem but I decided to tinker with the thing a few months later to lower the door down the remaining 2 inches. I began by pulling on the emergency release cord which detached the door from the moveable chain mechanism. The door now sat on the ground. I made a few mistakes here and there but long story short, I had to resit the chain on both the garage door opener sprocket and the opener's complimentary pulley unit on the opposite end. I tried opening the door, all seemed fine. On closing, the door went all the way down to the ground. Excellent.

The problem now is there is this incredibly loud stuttering-banging noise as the door is being "lowered" to the ground (on opening, the lift is smooth and quiet). On closer inspection, the trolley, rail, and chain begin to shake as the garage door is being lowered; the trolley appears to teeter up and down slightly as it makes its way down the rail. The header bracket (the apparatus fixed to the wall above the garage door) appears to bend up and down as the trolley moves towards it. The shakes begin once the trolley reaches about a quarter of the way down the rail.

On closing, when the trolley reaches 1/4 way down the rail, the shakes begin for the rail, trolley, and chain (the noise of the chain bashing against the rail is the most noticeable). Once the trolley reaches the halfway point, a much more noticeable stuttering-banging noise occurs (I'm guessing it's the shakes of the earilier parts having more of an effect on the segemented garage door and gravity).

The vibrations and noise are driving me insane. How do i go about resolving this?

Thanks.