Hello, I just recently purchased my first X (08, 4WD, 190k miles). Everything seems solid other than the notorious vibrating idle (haven't been to Nissan to attempt idle air volume relearn yet) And this annoying "whine" under light acceleration. While I'm by no means a master tech, I was a tech for Nissan for a short time and I dont recall ever coming across this is any X's, pathfinders, or frontiers. While driving at say 30 mph, ill give the vehicle a small amount of gas, just enough to maintain speed on a slight grade, and i hear an oscillating whine, or whirring. It goes away as soon as I release the gas pedal. Transmission fluid level/quality is good. T case fluid level/quality is good. Rear differential fluid level/quality is good. U-joints are good. Some fluid looks like it had been slung out over time where the drive shaft enters the transmission, but it isn't a large amount (trans output seal beginning to leak maybe?) Exhaust shields are all tight. I can't tell if the noise goes away at higher RPMs, or if it is just drown out by engine noise. Vehicle is currently free of SMOD, but it possibly could have been a cover up (no evidence to suggest that, just trying to narrow down all possibilities). The radiator appears to be aftermarket, but i did the bypass jist to be safe. I want to put an additional external cooler on, but do not want to strain a potentially damaged transmission. Other than the noise everything is perfect. Vehicle pulls hard, shifts smooth, and does not clunk when shifted into drive or reverse. I am curious as to whether not anyone else has experienced this, and what you did to remedy the problem. Thank you for reading!