I own a 2014 Mitsubishi Lancer SE 2.4l.

To begin, my car had a po171 code that was just recently fixed. And now it keeps throwing a po456 code. I had an issue in the past year at times where after driving my car at highway speeds for about 30 minutes, it would start making a loud vacuum like noise when accelerating and slowly diminishes when letting off the gas. With the sound comes a very noticeable lack of power. When this happens if I drive slower the sound and the lack of power doesn't seems to happen or very minimally. 

I have had my car checked for vacuum leaks and nobody seems to find any. Another note to add is that this seems to happen more so when it is hot outside, but when it has happened the temperatures on my dash seem to be normal. I also usually use higher octane gas usually (don't even know if I need to) but this issue started happening again today when I went to drive an hour and filled with regular gas.

I don't know a whole lot about cars but I have been battling this issue time and time again, and can't pinpoint on why it comes and goes. 

Thank you in advance, Andrew