I have a 2013 and had a similar issue, covered in the "whining above 70 mph" thread linked above. I'd recommend taking it to a shop who can put it on a lift and use a ChassisEar to listen around for the noise.
Mine also occurred at 70+ (only while coasting) and sounded almost electronic, as well. The sound drove me crazy for three years before I bit the bullet and took it to a reputable shop. They had it diagnosed in a few hours - worn pinion gear in the front diff. Mechanic said the noise can manifest in different ways depending on how the gear is worn, while coasting (like mine) and also while accelerating (like yours).
A worn pinion gear in mine is odd as it's relatively low miles, has had all routine maintenance and fluid changes at the dealer, and has been 100% an on-road vehicle from Day One, 80% of miles are highway miles. So nothing about its history would send up any red flags for a worn out front diff, but that's what it is.
Interesting observation in my research over the years is every post I've been able to find online about a GX owner experiencing this sound seems to come from a 2012 or a 2013. I am wondering if Lexus used a bad batch of front diffs around this time that for whatever reason experienced excess wear quickly due to the manufacturing defect. Of course these vehicles are all out of warranty now, and it sounds like the problem only affected a handful of them over a relatively short production run, so Toyota doesn't have to worry about it.