My bet is the coupler at the transmission end of the driveshaft. (as has been mentioned above a couple times). I'm not sure if Malibu uses double taper driveshafts, spline on the transmission end, or slide in coupler with key and square head bolts to hold. If it is a double taper, I would take the four bolts out to separate the driveshaft coupler from the transmission, slide it back and put a socket on the nut inside (just like on the prop end). See if it moves any when you re-torque it. You might be able to test this theory by running the boat first. Try taking off hard, then slowing down really gradually so the prop is always pushing, no water pulling it backwards from stopping. Hit it hard again and if there is no pop, you are definitely dealing with something loose in the drive train, most likely the coupler at the transmission end since you have already addressed the prop end.