If you get the freewheel off, check the inner body slop (side to side play). Some Chinese freewheels I've seen, came out of the factory with no second row of small ball bearings and this can cause grittiness. And you may be grinding the retainer ring on the outer face of the freewheel body. This face usually has two small opposing holes for a pin spanner. If you have a pin spanner that fits, check that ring and see if it's loose. Usually, it's left-hand threaded. I;ve rebuilt two of these by replacing the tiny balls inside with fresh ones and repacking. Both freewheels have been riding for over a year, one is almost two years I guess. Still in good shape after the rebuild and no grinding either. So you may want to check your freewheels. I used to assume freewheels were a staple commodity and just worked. But there were in the days of Suntour, Shimano, Sachs, etc. These days, I see brands like Sunrace, Falcon, and some no name stuff. I even have a single speed FW from India and it grinds on one gear. I got a replacement, also made in India for $8 and it grinds too. So I splurged and now I stock name brand Shimanos or ACS and pay $20+ and these are made in China or somewhere else cheap, but they don't grind and are smooth. Go figure.