Last year, after driving a long distance (about 200km), my 2013 Kia Cee'd started making a high pitch humming noise. It was loud when I was stopped but completely drown out by the engine noise as when the car was moving. While driving, the noise could still be heard very faintly with radio and AC off but didn't seem to change with speed or RPM.
The next day, the problem had disappeared. Yesterday, I drove again a long distance and the same noise came back. I investigated further and the noise is coming from the back of the car. It can be heard the loudest next to the back wheel wells.
I suspect it might be a small leak in the exhaust but I'm surprised it doesn't get louder with rpm. Does it makes sense that it would only be heard after driving a long distance ? What else could make noise around the back when idle in a FWD car ? Should I be worried ?