This has only begun to occurr very recently.
There is a feint flapping sound coming from the engine area when travelling at high speeds (100+ km/h) on the highway. It's so feint that I have to turn the radio down to hear it.
It occurs at random intervals and gradually increases over the journey (2.5 hrs on the highway).
I had the problem of poor acceleration yesterday, but today the acceleration was fine.
Could this be pinging?