Sounds like little marbles in the top of the jug...only happens under load?
I'm not a mechanic. But I'm with OhioPatriot. That could be pinging. I've had it before when I run 87 octane, it's hot out, and I'm pulling hard uphill. And as the weather warms up, it might seem to get worse with each ride...because, doesn't pinging get worse as it temps increase?
33 mpg doesn't sound out of line to me if you've got cams and you're pulling hard uphill.
2013 Cross Country (Houston/Katy Texas)
People think I am too patronizing. (That means I treat them as if they're stupid.)