Hi there, haven't posted for a while but I have a question that maybe you guys have an answer to. My sister has a Pontiac bonneville with 160k on it and its a 2000. Has the 3.8 and whatnot. It has been poorly maintained, if at all for the last couple years of driving. She is having an issue at the moment where when the engine is under stress, such as hard acceleration or going up a steep hill, the car begins to shake as if the RPMs are bouncing about and then there is a thudding sound. Not an engine knock but actually as if something is hitting the underbody of the car. Last time I heard a sound like that my cv shaft was flailing. Any ideas on what could possibly be wrong with it? It has me stumped.

 

Check motor mounts & trans mount.

 

I'm with Jerry on this, look at the mounts. The Bonnies have a small rep for having a weak lower engine mount (the upgrade method I've read about involves hockey pucks lol).

 

My Parents had a 2000 Bonnie until it was totaled W/ 178K on it.

Same issue came up around 170K in theirs, and it was a motor mount. Not exactly sure which motor mount though.

 

i also agree check the engine mounts