Long story short...my yukon started making a metal thumping sound. Went to tranny shop and he insists its the planetary gears just by hearing it. Not test driving it...nothing. It doesn't slip, miss, no issues until this weird sound occurred and even with the noise it shifts fine, doesnt change on a turn either. It does it consistently when tires are rolling. Quiet when stopped even in gear I put it in neutral turned engine off and rolled it, the noise still occurs. Coming from tranny area.