Grinding noise in 1st and reverse

[spookybathtub]

My manual Yaris is making a grinding sound at low speeds in 1st gear and reverse. It happens when I start moving from a full stop, and the noise stops once I get up to about 5 mph. It doesn't seem to matter if the engine is hot or cold. This has been happening for several months, but when I made a road trip to Texas last month, it seemed to stop, and now it's back.

About 2 months ago, I flushed the transmission fluid and filled with MT90. That made shifting a bit smoother, but didn't help with this noise. It sounds like it's coming from the engine bay, but I don't know what to check for.