I recently had a similar noise when I installed a new handle, which, I assumed was caused either by the new seatpost required for the handle, or, the changes in the seat.
After checking for the usual possibilities (including bearing holders) I spent 2 hours messing around with the seat/handle at the end of which, with no solution in sight, I again tightend the bearing holders and the noise dissapeared.
Details here-
Might be worth you re-checking the bearing holders, I wish i had