I remember reading a few years ago, that it's possible for the actual bulkhead to crack and start flexing.
If I recall there were a few posts on Saab Central about it, and it only seemed to be from guys in the UK.
This suggests that it has something to do with it being RHD.
I remember in the posts that someone had removed their engine to weld up the crack(s).
99 Saab 9-3se Imola Red
90 Saab 900 SPG Talladega red
88 Saab Powered Jeep Wrangler - 1999 Saab 2.0l T5
04 Yamaha YZF600R
00 Polaris XC700
99 Saab 9-3-RIP
98 900s N/A-RIP