[jjd]

After only 900 miles, my 2008 GranTurismo has developed a howl from the rear end. It starts at about 70-74 mph, gets really pronounced at 75-76 mph, then fades out competely by 80 mph. Only happens on the freeway after a 75-80 mile run on back-country roads (spirited but not hard driving).

I took it in to the dealer's service dept. after a run yesterday. The shop foreman and I took it out. He thought the sound was coming from the differential, and I left the car with them.

Today the service manager called, saying the differential is their prime suspect. They'd checked the oil in it, which was discolored and seemed burnt, but it wasn't low. It's a special type they have to order and won't be in for a couple of days. Then they'll replace the oil, drive the car and see what happens.

So far they don't have a theory on what the problem is. I'd appreciate any thoughts on it.

 

[bugattitech]

The best way I have found to deal with noises like that is to put the car on a lift and have someone get it up to speed. Then with someone underneath listen for the noise with a mechanics stethescope. This will isloate to the exact area. I have had differental noises before and they turned out to be wheel bearings.

As far as changing the differntal oil. I would ask how and why it got the way it is now. Was it under filled from the factory. You have been driving this car for how long with this noise. I would be worried about permnant damage. They should filter the oil when they are draining it to check for metal shavings. I would suggest asking questions such as how and why.

 

[Glenn Wallace]

This is probably a ring and pinion mesh (or heat treatment) issue --- you can get them lapped but on a new car they should just replace the entire diff. If the oil got discolored once, it is going to get discolored again.

 

[wachuko]

This is probably a ring and pinion mesh (or heat treatment) issue --- you can get them lapped but on a new car they should just replace the entire diff. If the oil got discolored once, it is going to get discolored again.

+1 I would be asking for a new diff...