The noise makes it sound like you're beyond my solution and in need of repair. But, on the old 1480s (one of which had the tranny apart 5 times and the gearcase replaced once) the solution ended up being to keep the shift cables and linkage clean at the transmission. If chaff and debris was allowed to build here it acted just like a spring, eventually pushing out of 2nd. We also put 915 detent springs on the 2 shift rail in these transmissions. They were stouter springs. I'm not sure what the 21s use there, but haven't had this problem with either the 16 or 21.