No, unfortunately this is not the beginning of a funny joke. It’s an indication of a problem with your water heater, though, if that’s where it’s coming from.
A tank water heater system may occasionally make this noise, and while sometimes it may be the symptom of something fairly benign that just requires a “quick” fix, other times it can be more serious. The fact is though, no sound coming from your water heater or any other home appliance should be ignored. A knocking noise very often indicates a problem that can only grow larger the longer you neglect it.