'99 Grand Prix 3800 V6 Thumping feeling

I own a 99 Grand Prix GT with 159,000mi and have been experiencing a low thumping/rattling noise coming from the front end of the vehicle. This "thumping" is felt in the steering wheel, accelerator, and the floor panels on both driver and passenger side. And in some circumstances can be heard; it is never extremely loud, but it is always noticable to either the driver or front passenger.

It only happens in the 50-60mph range and typically only happens when I let off the accelerator, change lanes, and turn slightly to the right. Although, after a certain point with turning to the right, the noise/feeling subsides, unless I let off the gas. But, as soon as I apply the accelerator the noise/feeling stops. It is also most audible when I plainly let off the gas- if that is all important.

I also experience a shaking of the vehicle when accelerating from a stop, and the faster I accelerate the more violent the shake- and I'm wondering if this contributes to the thumping feeling/noise.

And this, thus far, is what I have replaced...
3 of 4 wheel bearings
all 4 rotors
brakes
passenger inner tie rod
rear strut
and something to do with the front axle

I also have had 4 new tires put on and rotated.

I have brought it to the garage twice for this, and the 1st time they replaced the inner tie rod, and the second time they replaced, or fixed the axle (unsure about what specifically happened there).
And each time has not rendered the issue, it only quieted it for a short amout of time.

I would greatly appreciate any input, because this is getting quite bothersome and tiresome.

Thank you,
Stephanie

I would check the lower control arm bushings next, followed by the other axle that they didn't work on (or if they just replaced the CV boot, I'd say the axle is the primary issue). I'd also double check the front hub bearings, especially if the one on the right front is one that wasn't replaced (you stated 3 of 4 were changed). If the hubs are Chinese made, or the person that installed them used an impact wrench on the axle nut and/or didn't torque the axle nut to the proper spec, it's very possible one of your new hubs is your issue.

no body stated the obvious yet, see if the tires are balanced that may be causing the thumping /shaking

as i say cheapest fixes first

there's no better felling than spending $3500 to get all of your fuel injectors replaced just to find it was a clogged fuel filter

Thanks everyone for your input!
I greatly, greatly appreciate it!

I'll see if those things could be the case.

Oh, and to answer your question, yes, the tires are also balanced, too.
But thank you too!!

It could be the rotors too even new rotors,warped or the coating on the rotors has not been removed with brake clean.I have seen this with new rotors being warped

Too bad you rushed off and spent top dollar for possible shady mechanic work..could all be from a bent rim and/or poor wheel balancing.

Ok lets review, as this thread is getting convoluted and is all over the place.



I also experience a shaking of the vehicle when accelerating from a stop, and the faster I accelerate the more violent the shake- and I'm wondering if this contributes to the thumping feeling/noise.
Stephanie

Since it's only vibrating while accelerating, we can pretty much rule out brake issues. Since the vibration gets worse the harder she accelerates, it sounds like a driveline problem.


And this, thus far, is what I have replaced...
3 of 4 wheel bearings
all 4 rotors
brakes
passenger inner tie rod
rear strut
and something to do with the front axle

I also have had 4 new tires put on and rotated.
Stephanie

Although I try not to assume things, lets do so on the fact that the tires are new and were most likely balanced properly, because the new tires didn't help the issue.


I have brought it to the garage twice for this, and the 1st time they replaced the inner tie rod, and the second time they replaced, or fixed the axle (unsure about what specifically happened there).
And each time has not rendered the issue, it only quieted it for a short amout of time.

Thank you,
Stephanie

I think the 2 things you should concentrate on are finding out what they did with the axle, and if the hub bearings were installed properly and what brand they are. I personally feel it's more driveline related and not the hubs, but there's an outside chance they could be the issue so that's why I'm mentioning them. If they only replaced a boot and regreased the joint, then I would say the axle is bad and the cause of the vibration. If they replaced the axle, it could be the other axle that's causing the vibration as driveline vibrations can travel side-to-side. If the CV axles check out, it could also be an internal trans issue like the differential.