My guess is torque converter. I believe in stock form it will lock at ~40 (or 39) mph under light load. Would explain the sudden drop out of the sound. And it's coinciding with the drop in RPM.
The converter won't lock at all if the trans is under 105 degrees. Perhaps test this when the car is cold.
2009 PBM GT
Under the Knife Again
Wiseco Pistons, K1 Rods, Johnson Lifters, BLP Keep Hatin' PDS Cam, Roto-Fab, BLP Ported Intake and Throttle Body, Kooks 1-7/8 long tubes, XForce Varex Catback, ZL1 Converter, Camaro Trans Pan/Filter, GXP Brembos
Breyton GTS-AV Wheels, 6k HIDs, Holden Steering Wheel and Paddle Shifters, Eibach Springs, 35% Tint
Tuned by Day Tuning