 I purchased a brand new Elantra 2018. there is a knocking noise when accelerating. took it to dealership. they advise me to use good gas stations. they mentioned to" avoid certain gas stations" to see how it goes .There is also sometimes a clicking noise coming from the fuse box in the engine when vehicle is in idle.  I will take it back to the dealership. My previous vehicles were Toyotas. No issues with them.