The horn should have splines on the inside of it. If there is none or the splines are to many, you'll have to find the proper insert that matches how many splines your servo has. Most new horns come with 3 other inserts numbered with 23, 24, and 25 splines. Find the one that fits your servo. Perhaps a fellow racer/hobby shop could help you with this. The sound you hear is probably the servo splines stripping inside the horn.