Q: air conditioning on 2001 Cadillac DeVille DTS

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
the straight on vent opens once in awhile after a clicking noise , all other locations work fine
(2) Answers
You have a Mode door actuator going bad under the dash the noise comes from the little plastic gears inside the part trying to open or close the vent doors. It's a pain to change and the parts about 100 dollars. The door bracket arms need to be in the right place when you put the new part on. You better take it to someone who has done it before.
You may have a mode door actuator that is acting up. If so there should be a fault codes stores in the A/C control module which can be read with a scan tool. If the actuator is faulty it should be replaced.