2012 S40 2.0 Petrol - engine straining/loud at higher speeds

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I’ve recently bought a 2012 S40 2.0 petrol. Everything is fine with it in normal driving but on a recent long journey, I found it very loud and strained at higher speeds and revs (75+ mph & 3-3500 rpm). It wasn’t the smooth, comfortable cruiser that I was expecting.

It’s had a twin pipe aftermarket exhaust fitted which does make it louder at lower speeds but it’s smooth to drive.

I’m also seeing pretty low MPG of mid 20s.

I’ve plugged in the Torque app and I’ve got no faults showing but am getting some unusual readings on a couple of the O2 sensors as far I as I can read.

Could faulty sensors cause what I describe? Or another thing I read that may be causing an issue is the Swirl flap controller that’s located on the right under the engine cover.

Any advice or anyone had similar?