Rumble Under Acceleration (dont think it's thunk in trunk)

**NoNewGoatYet** · 05-23-2014, 01:50 PM

Hi all, first post here! I've been periodically browsing your forum here for a while and trying to learn about ls's, I'm an old fart (43) and have only played with antique toys mostly, 1969 - 1971 era poncho stuff. I'm almost ready to take the plunge and buy an '04 GTO. I realize by now most of them have a good chance of being beat on pretty hard, so I have some concerns and I'm not really ready to write the check quite yet. Anyway, this one I've found has 106,000 miles and seems to be fairly unmolested. The only performance mods are exhaust and cold air intake. It has the manual transmission and I've got a concern with something. I've read about the "thunk in the trunk", but I'm not sure that's what I'm hearing/feeling. Under acceleration (not even hard acceleration, nothing that would be close to causing wheel hop or traction loss), there's a noise coming from the back of the car. It's like a fast vibration, reminds me of hitting the rumble bars on the interstate. I wish I had the car with me (it's on a lot) because I don't have an idea of RPM's, duration of the issue, etc., but I can say it's in 1st and 2nd gear and begins toward the bottom end of the power band. Does anyone here have any experience with this? I don't want to buy a headache...

Thanks in advance for any input!

**pajeff02** · 05-23-2014, 04:52 PM

Welcome to the site from an even older fart.

Could be a u-joint in the driveshaft or a CV joint on an axle. Hard to tell -- get the car up in the air and check things out if you can.

**cammed goat** · 05-25-2014, 06:14 AM

Find out if the gear oil in the diff has been changed. Our cars require a friction modifier that MUST be added to the gear oil or else the differential will chatter.

Factory uses 75-140 synthetic gear oil but some run 85-140 either synthetic or regular.

For friction modifier, most GTO owners will advise using the Ford Type F friction modifier.

**NoNewGoatYet** · 05-25-2014, 07:10 AM

Thanks for the help! I'll definitely tuck the friction modifier information away for when I do get one of these cars. Might be now, might be later...

I told the dealer I wouldn't purchase with this issue. Apparently, he had it looked at and the cause is a bad driveshaft hanger bearing (according to the dealer here, I don't have access to the car). Mechanic said he could grab the driveshaft and move it up and down freely. Does this make sense? It seems questionable to me - I'd think the thing would be banging around all the time if this is the case. The bearing is getting replaced later this week, and I'll post here if it fixes the problem.

**cammed goat** · 05-25-2014, 02:14 PM

Carrier bearing is shot. The GTO has a two-piece driveshaft which is coupled together with a center support bearing. The bearing is encased in rubber which tears over time being normal driving or hard launching. Mine went and I ordered a custom-made one-piece aluminum piece from The Driveshaft Shop. Hopefully the dealer will also replace the rubber isolators at the joint between the trans and differential.

**NoNewGoatYet** · 05-26-2014, 07:26 PM

Yep, I expect it's the carrier bearing. It sounds like this might be a tough pill to swallow for the dealer, am I correct it's a "non-serviceable" component? I see there are some out there, but I bet they're going to check with the parts counter at the dealer and find out it's a non-stock part and start scratching their heads. I think I'll call tomorrow and see if he'll drop the price and I'll install a one piece driveshaft myself. The risk would be that doesn't fix the problem and I'm stuck with something else wrong after I buy the car.

**cammed goat** · 05-27-2014, 06:22 PM

The driveshaft and differential are non-serviceable. I have read that the carrier bearing "could be" replaced, but the vibration afterward really defeats the purpose of saving the factory unit.

**2002ssslp** · 05-28-2014, 09:16 PM

Check the half-shaft bolts on the rear, they get lose and need to be tightened every 25k or so unless you use loc tight. You will need to torque both sides, diff and hub side.

**NoNewGoatYet** · 06-05-2014, 07:40 PM

Update: I bought the car and had the chance to crawl underneath and sure enough, the carrier bearing support is shot. I ordered a one piece driveshaft and will hopefully receive it early next week, maybe even tomorrow, get it replaced, and fix the problem. Half-shaft bolts look good, thanks much for that tip. I'll let you know how the shaft works.

**pajeff02** · 06-06-2014, 01:51 AM

Congrats!

**cammed goat** · 06-07-2014, 06:01 PM

I think you'll be quite pleased with the one piece. Lighter and you'll gain some hp/tq.

**NoNewGoatYet** · 06-12-2014, 06:57 PM

Just got the one piece in, and it fixed the problem. Now I have the thunk in the trunk, but hey that's okay - it drives a TON better. There used to be a slight vibration at 55+ mph that's now gone, and no more rumble bars under acceleration. But the TSB issue is clearly present in my car, I hadn't romped on the gas before this so didn't experience that before tonight.

One note: I had to loosen all the exhaust system bolts (it has an SLP system) and push the system toward the passenger side of the car, hold it, and tighten. On the first test drive I found the clearance between the right side front exhaust hanger and the shaft was too tight and there was a clatter. Seems to have enough room now. We'll see how things settle in.

Thanks for the help!