Sounds exactly like the problem I had with my Rizomas. After analyzing the situation, I'm pretty sure the sqeak comes from the washer that goes between the bolt and the rearsets. The friction between the two causes the squeak. I tried greasing the washer really well, but after a few minutes of riding, the grease would get worn out and the squeaking would start all over again. I even tried polishing the washer and that didn't help.
My solution that has worked so far is to locktite the bolt and use less torque. I marked the bolt positon with a pen and check it often and it hasn't moved in the last week.
I think that the difference in the metals between the aftermarket rearsets and the stockers might be the problem. The aftermarket rearsets are probably made of a harder material, causing wear against the washer, causing the squeak.
My solution that has worked so far is to locktite the bolt and use less torque. I marked the bolt positon with a pen and check it often and it hasn't moved in the last week.
I think that the difference in the metals between the aftermarket rearsets and the stockers might be the problem. The aftermarket rearsets are probably made of a harder material, causing wear against the washer, causing the squeak.