Ann W Loudon , TN May 30, 2018 Thermador 36" downdraft vent for cook top made a thunking noise when lowered Found a knowledgeable helper who took the unit apart and replaced the Drive Arm Assembly and Drive Link. We had also replaced the switches and motor a month before, when we then noticed that the unit was not retracting at an even rate of speed. I'm estimating that it took 1-2 hours because the unit is in the middle of a kitchen island. We needed to remove and replace storage drawers to access the unit, which took as long as the repair itself. The old c. 1998 Drive Arm Assembly was very worn in the large hole with the square top (on the right end in the picture). This seems like a weak point in the design, as the metal wears after about 5 years of use, causing uneven drop (sudden plunge) when lowering. I suggest that if you don't do this very often, get the service manual and make notes of what you did, as you may have to do it again in a few years. Read More... 2 People found this story helpful Do-It-Yourself Rating: Repair Time Estimate: 1-2 hours Tools: Nut Drivers, Screwdrivers