Thread: 2008 Pontiac Grand Prix unknown knocking noise

Hello. I've bought a 2008 Pontiac Grand Prix 3.8L with the 4T65-E transmission and 145k miles on it. At the short test drive i noticed a knocking noise while going around right corners that could also be felt in the steering wheel. You start hearing it at 25 mph and it gets faster with the speed of the car (Not with the RPM).
I thought it's just going to be a bad C/V axle or wheel bearing and bought it regardless. I let my shop test drive it and he immediately said left side C/V axle. So i bought one and brought the car to the shop to get it replaced. Today they called me and told me that they didnt replace the C/V axle, they believe the transmission is bad. They had it on the lift and claimed the noise came from the transmission and not from the C/V axles. Saying its a bad transmission and i should just drive it til it breaks, its not worth fixing.

Here is the weird thing though. The noise is very loud in right corners, slightly audible while driving straight (around 50 mph) and a bit more audible in left corners. But again very audible in right corners and you can feel it in the steering wheel. But the knocking is also louder, audible and you can feel it when you brake a bit harder and come to a stop. Almost like a bad brake rotor, but it's the same exact noise. I don't know how that's supposed to be connected to the transmission.

I do have a video but i need to find a way to upload it somewhere.

I know without the video its kinda hard to tell from a description where the noise could come from but are these transmissions known to have issues like this ? Did my shop just tell me some BS because they didnt wanna deal with it ? Or is there a way how i can check if its coming from a C/V axle, or wheel bearing, or if its really a transmission problem. If so it could only be either the differential or output shaft right ?

Thanks for any help

Originally Posted by Woolsa

Hello. I've bought a 2008 Pontiac Grand Prix 3.8L with the 4T65-E transmission and 145k miles on it. At the short test drive i noticed a knocking noise while going around right corners that could also be felt in the steering wheel. You start hearing it at 25 mph and it gets faster with the speed of the car (Not with the RPM).
I thought it's just going to be a bad C/V axle or wheel bearing and bought it regardless. I let my shop test drive it and he immediately said left side C/V axle. So i bought one and brought the car to the shop to get it replaced. Today they called me and told me that they didnt replace the C/V axle, they believe the transmission is bad. They had it on the lift and claimed the noise came from the transmission and not from the C/V axles. Saying its a bad transmission and i should just drive it til it breaks, its not worth fixing.

Here is the weird thing though. The noise is very loud in right corners, slightly audible while driving straight (around 50 mph) and a bit more audible in left corners. But again very audible in right corners and you can feel it in the steering wheel. But the knocking is also louder, audible and you can feel it when you brake a bit harder and come to a stop. Almost like a bad brake rotor, but it's the same exact noise. I don't know how that's supposed to be connected to the transmission.

I do have a video but i need to find a way to upload it somewhere.

I know without the video its kinda hard to tell from a description where the noise could come from but are these transmissions known to have issues like this ? Did my shop just tell me some BS because they didnt wanna deal with it ? Or is there a way how i can check if its coming from a C/V axle, or wheel bearing, or if its really a transmission problem. If so it could only be either the differential or output shaft right ?

Thanks for any help

A collapsed engine or transmission mount maybe?

You can check the hub bearing easily. Just jack up the front end until both tires are off the ground and then put your hand at the 12 and 6'oclock positions on the tire and push with one hand and pull with the other. Basically just try to rock the tire. If you have play then that hub is shot.

Also worth checking the trans. fluid level. Check it while the trans is fully hot (after a long drive), in Park with the engine idling. Make sure it's in the crosshatches on the dipstick. That trans should use Dex-VI IIRC.

Hello and thank you for the replies. I made a video, here you can listen to the knocking sound it makes. As you can hear its a speed and right turn dependant knocking noise, that also appears louder when you hit the brakes.

https://cdn.discordapp.com/attachmen...0282/Knock.mp4

I thought that with my 08 Grand Prix changed the mounts , axles …. Wasn’t that … It is was loose radiator support/ subframe bolts . Mine did it for the longest time ever..

Loose radiator/ subframe bolts