2007 MT Fit Sport, no Mods, 110K miles, original clutch and driveline

Upon moderate to hard acceleration in a straight line, there is an odd non-metallic popping/clicking sound coming from what appears to be the rear of the engine. Happens hot or cold but is only 50% consistent. No work recently done.

???

 

Does the sound increase and decrease with rpm?

If you haven't checked them recently, I would check to make sure the spark plugs are tight.

 

Yes, sound increases/decreases with RPM. I installed new plugs summer of 2016 using a torque wrench.

Performance is good and unchanged.