What causes this popping in my elbows when I do bench press?

[Einhander12]

I hear this little pop when I do a Bench Press. It's usually the left arm and it happens on the way down. It's been this way for quite a long time. It isn't painful, but I do wonder what I'm supposed to do. No matter how narrow or wide or what kind of form I use, it doesn't go away. But it doesn't happen on Lat Pulldowns or other exercises. Mostly bench press and shoulder presses.