Or search: barm, yeast, leaven, imponderable
2 Syllables: SU
deadening, deafening, leveling, meddling, reckoning, settling, threatening, wrestling
3 Syllables: SUU
beckoning, bettering, editing, entering, Evelyn, festering, lessening, lettering, levelling, measuring, menacing, questioning, reddening, revelling, severing, threatenings, trespassing, weathering
3 Syllables: USU
endeavoring, unsettling
4 Syllables: USUU
developing, discrediting, embarrassing, endeavouring, enveloping, inheriting, unquestioning

Found 35 words across 4 groupings. Skipped 1 excess word of a similar stem.