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It is not hard to replace the differential fluid. Remove the drain plug (see picture below) and reinstall when the fluid stops draining. Then remove the fill plug and fill it to where the fluid is just running out of the fill hole. Honda has a TSB for a noise in the rear differential on turns and the fix is to replace the fluid. If that does not help then the clutches in the differential will need replaced. You will need to get the differential fluid from Honda. It is Genuine Honda Dual Pump Fluid. It has added additives in it to keep the clutches lubricated and clean. Without these additives it is going to make more noise and possibly ruin your differentail clutches.