Weird grinding shudder sound

Hey guys, tonight for the first time my 2011 Taco started making a weird sound when decelerating. It almost sounded like an 18 wheeler downshifting going down a hill. That loud grindy, winding down noise. It wasn't anywhere near that loud - but that's the closest thing I can compare it to. It happened twice. And when I slowed down and took it slow, I made it all the way home without hearing it again. When it was happening, it sort of felt like the truck was driving through sludge, like it was struggling. And after about 5 seconds, it went away. Any ideas?