Clank clank clank is the mechanical version of Plap Plap Plap.
You should also be able to see a video with...uh .... Freddy Fazbear featuring it.
That has a visual aid of you still don't get it after that then you probably need to have a good long talk with someone about the birds and the bees.
Oh no no no, I understood it after looking it up, although I wasn't expecting to actually find an answer to that on the urban dictionary, what I didn't get is what Herta was trying to say in the last panel with all the "clank clank clank" not what the wording meant.