I think it may have something to do with the prop your using.  A 13 x 8 is way below any recommended prop.  I use a 16 x 6 on my 91FX.  The only time I get that "cackle" is when I level off after coming out of a dive where my throttle is pulled way back and the prop has been acting as a brake.  When I bring the throttle up 1/2 way or so and the engine is firing, but still not working very hard it cackles.

I'm not sure what the technical term is, but I think your engine just isn't working hard enough to spin that prop, and it's causing some sort of detonation issue.  Try a bigger prop, at least a 15 x 8 and see if the engine likes that better.