Sounds like his car engine is knocking.
What does that mean?
It mean's something is wrong inside the engine and he should probably take it to a mechanic.
You've got good ears to notice that something is wrong with his car. Noticing that something is wrong with an engine is a great skill to have so you can keep them properly repaired and working.
Damn car owners are clueless. I'm the mechanic.
It sounds like you're good at your job, but it's important to remember that most people have no training, and little understanding, about engines.  It is important to pass your wisdom along whenever you can.  If people take better care of their cars, that could actually mean more lucrative work for you.