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I guess the PCM is a powertrain control module that can shift the transmission with electrical signals. My guess it that might make the same noise if you had the PCM do the switching between neutral and high gear without using the shift lever. If you check the signals going to the transmission you might find out if it is trying to go directly from neutral to high gear or engaging a lower gear first. I know that old Mercedes transmissions (without electronic control) first go to a lower gear when shifting from neutral to drive.
If I wanted to modify the system to shift to neutral when the throttle is closed, I would start by finding out what voltage levels are on the wires that control shifting when the transmission is in the various gears and neutral. Then I would build a separate box connected between the PCM and the transmission which would monitor the throttle position sensor or the MAP sensor or a vacuum switch and send the appropriate signals to the transmission for neutral when decelerating. I would automatically cancel using neutral if the shifter is set to a lower gear or the brakes were applied. I would also have a separate switch to turn off switching to neutral. I would either use relays or transistors.
I think that the GM overdrive transmissions use an overrunning clutch to eliminate engine braking in overdrive gear. It can transmit torque in one direction only.
It should be possible to add an electrically controlled clutch behind the transmission to automatically eliminate engine braking in rear wheel drive vehicles. I've never heard of anyone selling such a thing.
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