Sunroof knocking/clicking noise on 13 Toyota Highlander

ipt

my sunroof works normally but makes clicking/knocking noise when it first starts to open. As it first begins to move back , the sound at the beginning of motion. there is one single loud clicking/knocking. it seems like somebody knocking on my roof top above my head. the sound comes from the rear of the sunroof. Other than this noise, the roof works perfectly.

I took the car to the service. the guy said it is completely normal. he also lubricated rubber on the sunroof with some kind of spray can, but it didn't work. the sound still there. He said he couldn't do anything if the sunroof works normally. does anybody have same issue? maybe I worried too much?