Flattening rhymes with:
battening, fattening, battening, brightening, captaining, disheartening, enlightening, fattening, frightening, heartening, heightening, lightening, shortening, smartening, straightening, sweetening, threatening, tightening
Flattening sounds like:
faulding, faulting, feldman's, fielding, flatness, fleeting, flitting, floating, flooding, flouting, folding, fulton's