For the past couple of days I have been doing some maintenance on my 2003 coupe, 6mt, 138,000 miles. I replaced the fluids in both the differential and the transmission. Factory Nissan 75w-90 oil in the transmission and GL5 80w-90 in the differential. When I completed the job, I took the car out for a drive, all was well. The next day I took all of my wheels/tires off to inspect the brakes and clean out debris, buttoned them up properly and parked the car. This morning, as I was driving the car I noticed a howl/growl/whine from the rear of the car during accel/decel particularly louder below 50mph. After driving a few miles, I turned back at went back to the house to try to diagnose the problem. I checked the differential fluid level, was about a 1/2 half a quart low, probably because I didn't jack the car perfectly level, filled it up. I checked the transmission fluid level, it too was also 1/2 quart low, most likely for the same reason, filled it up. Test drove it, noise is still there. After browsing the forums, I took my girlfriends car to pick up some synthetic gear oil. I bought Royal Purple 75w-90 synthetic and swapped it out. Test drove it, noise is still there. I have also checked the wheel bearings, everything seems taught. Tires are fine, never made a noise before. My old 300ZX had a bad differential and it sounded very similar to the noise i'm getting from my G. I am not getting any performance loss from this problem. Let me know what you guys think is going on and what else I could try.

 

Would changing from 75w90 to 75w140 in the differential help? That's the only thing I can think of at this point, when Infiniti did the maintenance some time back, they used a thicker oil and now that I have changed it, it's causing this noise. I'm stretching here, any ideas?

 

For anyone who's interested I took the car to the dealership today and the problem was from the gears in the differential. Starting to look for a used one.