B Marques

I have a 2010 GL350 with 59k miles (did the trans service around 57k due to just purchasing truck). I notice only when slowing down and getting back on the gas it makes a thump. It shifts through all gears and downshifts smoothly. Only issue I notice is if I'm braking or slowing down and the stepping on gas. Weird thing is it doesn't happen all the time. Do you think it's trans /driveshaft/Guibo? Don't know what else it could be.