Lee said: My IO-320 does the same thing.  I've discussed it with several greybeard Lyc fliers among the local EAA group and they say it is "after-firing," as opposed to "back-firing."  After-firing is caused by a too-rich idle mixture, they say.  On their recommendation, I have started leaning while taxiing, and it has cured the problem.  I pull the mixture back until it starts running rough, then enrichen by a 1/2 turn.  Also seems to help with the mag check during run-up (i.e. the plugs haven't gotten carboned up as much as with the richer mixture).

Bob,

 

What Lee said!. 

 

Geeeeze, I forgot to mention that I lean my IO 320 with the mixture control just after the engine start settles down.  The same is true once I have left the runway after landing.  At my home airport, where I have years of taxi experience, the mixture pull is automatically done to a certain distance from the panel accompanied with minor adjustments, if necessary.

 

If that doesn't help, try the other things in my prior response.