Here's one for those that think they've heard it all. I have a thump in the passenger compartment of my '98 OBW 5 speed (2.5L) that I can replicate fairly easily when I brake hard or go over a speed bump at speed. It sounds like a small plastic door swinging shut near the firewall. Once i've "loosened it up", I can make it thump simply by getting on and off the gas or slowing down/speeding up. It really doesn't seem to affect anything other than my sanity though the car is very jerky and the constant thumping in stop and go traffic is getting old. Any ideas?

Check the "Dog bone " thatgoes from he fire wall the transmission , one of the bolts may be loose

 

SEA#3