Welcome to genvibe!
If it's a speed sensitive humming or growling noise, similar to what you might hear when a manual transmission Honda is travelling in reverse, it's likely a bad bearing in the rear differential.
If you are diy, it's a straightforward replacement, but it's also a lot of wrenching. You don't need to replace the whole differential- just the bearings inside the viscous coupling, which is 4 bolts for the drive shaft and 4 bolts for the cover. Wiggle off the viscous coupling and remove cover & dust shield (watch out for dripping diff oil). Tap off the old bearings, gently tap on the new bearings (front and rear), replace oil seal and dust shield, and reinstall the coupling and drive shaft. Top off differential oil and you should be good to go, as long as you don't break a cover bolt like I did.
There's another 100k miles in this car if it was taken care of.
If you are still thinking of selling, give an indication of where in the world she's garaged...