That's not rasp, that's knock. 2600-2800 rpm partial throttle loads the engine a decent bit and it will knock. The few times I've run 87 that's exactly what happened. Extended acceleration onto say an onramp, always that knock at 2600-3000 rpm. You may just hear it more now because the Y-pipe isn't muffling the sound as much, but I'd bet it's always been there even with stock Y.

 

You may be right about the noise being knock instead of rasp. The vibration type noise from the y-pipe was almost silent after I filled with Sunoco 93 and reset my ECU. I will stick with premium shell or Sunoco and reset the ECU regularly.
The stock y-pipes have insulation under metal jacketing probably designed to reduce the exhaust vibration noise. Some G stock y-pipes from other years use the hockey pucks too as vibration isolators. So makes sense that an aftermarket pipe could emit noise under engine load, especially with the stress from ignition knock and how the ECU adjusts the timing with low octane fuel
I recommend using the Venturi cone reducer when connecting a new HR ypipe to a stock sedan or coupe mid pipe.