It might be the alternator; but, the only thing to consider is that alternator noises or any noises originating from the drive belt area of the engine should be engine RPM related, not vehicle speed related. Alternator bearing noise should be 'repeatable' as you shift through the gears, not just appear at vehicle speeds above 85 mph.
If you have access to a mechanics stethoscope (or a suitable home fabricated bodge), apply the stethoscope to the frame of the alternator while it is running and have someone increase engine RPM while you listen. Bad bearings will usually reveal themselves with noises ranging from howling to scraping / grinding.
Do you normally drive with your AC in Auto mode? If so, the ECU will briefly shut off the AC when the engine is at high load (accelerating above 85 mph probably qualifies). It does this by disengaging the AC clutch allowing it to freewheel. If the AC clutch has some issues the noise may materialize when it starts to freewheel. This should be fairly easy to test by just forcing the AC compressor off all the time using the CCU controls and then going for an 85 mph test ride. If the noise appears and disappears just like before then it is not AC clutch related.
If it is an alternator problem. doing or having a rebuild done on your alternator is your only option because new and factory rebuilt alternators are un obtanium. However, make sure that it actually is an alternator problem because the noise as you describe is not 100% consistent with alternator bearing failures. The problem might be the tripod bearings in the right side half shaft or the right rear wheel bearing. Both of those make noises that are more related to vehicle speed, not engine speed.
My 2000 also had a noise that got worse as vehicle speed increased. It was the rear tires howling because of cupping caused by a rear alignment problem. My initial best guess diagnosis at the time had been a failing wheel bearing.
