Removed tile from a plywood subfloor and there is a very small ammount of uneven thinset residue like 1/16" in spots randomly scattered. Not worried about the level as it appear flat but worried when I screw the 3/8" underlayment down over the top before tiling that I'll get some crunch noise over time. I have a random orbital and angle grinder, its a small bathroom. Should I just hit it with some 80 grit or get one of the diamond head angle grinder discs. Seems like they may be overkill and I might make it more unlevel. Or is there a product I can apply before the underlayment? Not sure about self leveler poured between the layers cracking after screwing through. Other have suggested rosin paper 1 or 2 layers
I had a similar problem and I just ran a three inch cold chisel over the areas that were a problem. Granted, you might not want to do a large area like this but for a small bathroom, it worked great. No thinset dust to breath, it just shaved the residue off the sub floor and I swept it up. Just a thought. You could use a regular chisel but you'd have to put a new edge on it afterwards.