I have this sound from the engine when going up a hill, getting on it hard or pretty much the whole time while above 60 MPH. It sound like an octane knock, just not sure if anyone else has had the issue and was something different. I have checked the MAF and cleaned it, checked the gap on the plugs and now have no clue on what to do to fix it.
I think you have diagnosed the problem correctly as spark knock. Try a different brand or octane rated fuel, you could also add some octane booster to you fuel and see if it helps.
You could also have a bad knock sensor and if that's the case it is not telling the PCM to back the timing off a bit. Don't you just love these electronic marvels?