How to get that click noise off my iPhone?

I just got a used iphone, a 3gs. When i turn the screen off, it makes a clicking noise. How do I get rid of that, it's so annoying.

You can start buy selling the iPhone and switching to android.

Settings > Sound > *scrol down to bottom* then turn off the click noise

There are two ways. One is using the switch on the left side of the phone (This option turns of All sounds on the phone) The other is going to sounds in your settings app, and scrolling to the bottom and turn lock sound OFF.

Settings > General > Sounds > Lock Sounds > OFF

I like that sound.