All my T-Maxxes had a lean out problem.The 3.3 models were the worst..It became more worse when I added a fuel filter.Its due to poor quality control aswell as a poor fueltank design.

I modded the fueltank to overcome that problem by heating and stretching the fueltank output nipple more open aswell as drilling the puffer nipple bigger too...3 trucks had casting plastic slag partially blocking fuel flow..With an unmodded fueltank full of water it took 3 1/2 mins to empty..After modification it took a tad more than half the time to empty the same full tank of water...not to mention Ive never had a lean-out condition again plus they all stay in tune more uniformly throughout the whole tankful without having to richen/lean all the time..

The zipping sound could be your diff gear in the front/rear diff or bad bearing in one of the diff cases..It could also be a gear with a broken tooth in the transmission...

Your best bet would be to remove the transmission and open it up to inspect the inside...While you have the transmission out check the diffs one at a time by twisting the pinion gear forward and reverse via the driveshaft to see if its totally smooth or if you feel a defined gear mesh..If the gear mesh seems defined like it has detent stops and doesnt feel smooth then thats the source of the noise but Id definately open up the trans also to eliminate any doubts or other problems that could pop up..Good Luck