Every time i get on the highway once i push my civic around 60 to 65 it has a loud thumping noise. I dont know what it is. The oil is ok. The brakes are ok. I'm worried about pistons and such. Does anyone else have this problem? No exhaust problems. Whenever i run a code check it's always an air leak which i'm assuming is the purge valve stuck. Help please. Shes my baby.

 

What area of the car does it sound like it's coming from?