It is critical that the correct Honda lubricant be used in the differential. do you hear a chattering type noise on turns as well. Use "Honda Dual Pump Oil" its not expensive but again critical that "this" oil be used. If you are doing this yourself. Drive the car in a parking lot doing figure eights to get the oil hot. Drain the oil and refill with the special Honda oil. Has your car been making this noise for long or since having the differential oil changed by a quick oil change franchise?
Is your vehicle all wheel drive? If so, it could be your final drive in your front diff or transaxle. If you let off the gas at 40mph this sounds like a classic gear whine, however, how are your tires. If they are all-weather tread and get worn, thye too can make a noise that is similar to a gear whine when they are up to speed. Hope that this gives some input to your problem.