Recently my 914 has developed a loud squeak/thumping sound right behind the driver seat. It sounds like it could be coming from the suspension. It happens when going over bumps, and turning hard. When I hit a bump slow, its more of a squeaking or creaking sound, but when im driving and I hit a bump its more of a thump. When the car is rocked back and forth sideways, it also makes the sound.

I was thinking maybe trailing arm bushings, or even engine mounts? Any help would be greatly appreciated, thanks.

I had this too. I cranked down the top shock mounting nut and it went away. Maybe it would work for you. I also have poly bushings on my trailing arms. That squeek never fully goes away.