I had the TSB for the cam phaser done at my local Toyota dealer a few months ago. At the time I only heard the grinding on cold soak startup. Within the last few weeks I have heard make this noise after starting it up after driving for a good 45 mins on the highway. the dealer said that they have never had a comeback after performing this work on other cars. I want to to believe that's true. After watching a few other videos on YouTube about this I'm starting to think the sound reminds me of what happens when you hit start after the car is already running. Is it possible that the starter just isn't disengaging properly intermittently? It happens so unpredictably that it might be hard to just leave it at the dealer for days hoping it works. The car has 100k miles and 5spd manual.
Well, the cam phaser noise problem is definitely a COLD startup problem (went through this with my 09 Matrix), so I think you are likely to be right about it being a starter hang-up. The good news is that is not a terribly expensive or difficult fix.
Well I changed the starter finally. It's been almost 5 days now and no grinding at all. When I took the old Denso starter off the teeth looked smooth and shiny but had no signs of damage. I guess there must have been a small imperfection that kept the gear from retracting all the way.