Weird quick sound on my timeline that I cannot see or fix when I look at my audio

Hello, I have got a very weird, loud and short sound that comes up during one transition (but yesterday, it was during another part of the video, it seems to be moving). It doesn't appear or make that sound in my timeline but once I export, it comes up. For you to understand what I am talking about, here is the extract : weird sound

I have little experience in FCP. Maybe it is something well known. Thanks in advance for your help.