A change in sound when accelerating led me on a search to find out what was going on. Snooping around beneath the S4 revealed this:
![Apikol B5 S4 Transmission Mount](https://myaudis4.com/wp-content/uploads/apikol_bushing_troubles.jpg)
Found it! Fortunately Apikol sells just the bushings for the mount so I ordered a pair to replace the failed parts.
![Apikol Differential Mount Bushing](https://myaudis4.com/wp-content/uploads/apikol_blowout.jpg)
It turns out that there are two versions of this bushing so I needed to order both since I did not know at the time which version I have. I’ve documented the part number in case this happens again.
![Apikol Differential Mount Bushing](https://myaudis4.com/wp-content/uploads/apikol_failed_bushing.jpg)