Today, when I was getting my car tuned, when I would shift from 3rd to 4th gear it would grind going into gear. But it ONLY does it at high rpms. . . I have my clutch pedal adjusted and everything. . So is it the synchro?

 

It might be the synchro, but it could also be the clutch. When my ACT began to wear out, the first symptom was a sudden 3-4 grind that had never been there. Due to all of my racing, I first assumed my 4th gear synchro had begun to fail, but shortly thereafter, my ACT started slipping (after tons of racing abuse). When I put in the new clutch, the grind was gone and hasn't come back.