The exhaust is stock. It's definitely not exhaust noise. I'm leaning towards the gears as drbobvs suggested. Maybe it's the U-joints too...or the pinion bearing. This car has 98,000 miles on it.

 

If you're sure it's not exhaust and since you added that you have 98,000 miles it could be the differential. Have you checked the pinion bearing and looked for excessive backlash? A loose or worn bearing won't usually make noise under load but will upon deceleration.

 

Wow, thats alot of miles for a S197! Until I read this post I figured it was exhaust too but now I am confused!

 

This past weekend, I changed the oil in the differential. I purchased a lube-locker gasket and 3 quarts of 75w-140 Royal Purple synthetic oil. I pulled the rear cover off. It was obvious to me the original owner had never had the fluid changed. I bought the car with 96,000 miles on the clock. The cover still had the factory grey silicone sealant holding it on. I figured with 98,000 miles on it the original factory oil might have broken down. So, I had high hopes that a flush and clean would eliminate the sound. I did not check the backlash with a micrometer while I had the cover off. I did howevergrasp the ring gear and rotate it up and down to see if there was any slop. There really wasn't.

What was interesting though is that there was a black slime covering all the internal parts and surfaces. I had to use two large cans of brake cleaner to wash all of the black stuff off of the interior of the differential case, ring gear and back of the cover.

So, I buttoned everything back up and this morning when I drove in to work, I still had the noise.

 

Drop the drive shaft and check the pinion bearing for looseness and wear.

 

wow you bought the car with 96K.how much did you pay??

 

$13,500 cash.

 

dang an 06 with that many miles, where the hell did the guy drive? cross country a couple times a year? thats crazy (to me anyways)

 

The original owner was based out of Waco, TX and was in sales. That's all I know. I bought it from the dealer he traded it in to.

 

That means you have lots of highway miles and with proper maintenance those kind of miles are very easy on a car. I'm sure once you get your noise sorted out you'll have many trouble-free miles ahead of you.