Noise in the middle of the car when it's moving

Car: 2001 Forester S w/ 4EAT

Pertinent Modifications: None

Description of the problem: Whenever my car moves, in the middle of the car (Around where the back seat is) it sounds like someone is taking a hammer and smashing the car from under it. Like a loud "thump-thump-thump" at high speeds it gets really fast, almost like a jackhammer.

Recent maintenance or modifications:
- My driver side front wheel's lug nuts fell off at 80mph, so I bought new lug nuts and replaced it. The inside of the wheel itself was almost shredded.

- I installed the rear pair of rally armours yesterday. Which is weird, because the problem started after I installed the mud flaps.

 

Sounds like someone strapped something to your drive shaft.

 

Would the way that I jacked it up be the reason why it's making the noise? SF.org said to jack up the back of the car via the drive shaft.

 

Is the sound rotational? Is it once per revolution? Or does it sound like something that could be flapping/moving due to more drag at higher speed? The best thing you can do is get under the car and inspect it. Check heatshields, drive shaft, exhaust. Move things around, see what's loose.

Mike

 

It's once per revolution.

Oops, that's what I meant. My bad.

 

Center support bearing on the driveshaft could be shot. It's not serviceable but you can get used ones for pretty cheap.

 

Also I would not jack the car up by the differential either. I just don't think it's good for the bushings on the rear diff rear mount. They are intended to hold the differential not support half the vehicle weight. I think Subaru says not to. The manual will identify the proper jacking points.

 

I'd be curious to see how many people do. I've never not used the rear diff. Never noticed any issues because of it.

If you don't have a hoist and you don't jack on the diff, how do you raise the rear of the car at one time?

 

Been jacking my Legacy by the rear diff twice a year for 10 years now. Have noticed no ill effects.

 

Me too.

 

I remember reading somewhere that the rear diff was a jacking point?

 

we talking about jacking it in here?

 

always in the rear diff.

 

You'll never know

 

I've always raised the rear one side at a time from the sides near the rear subframe mounts. I jack my ford truck from the diff. After searching I have found no good reason not to jack from the diff. No stories of failed diff bushings from it. I went out and tried it and the diff moves a little before the car lifts up, but nothing bad happened. I stand corrected.

OP have you found anything loose causing your noise?

 

I think it is a flat tire. Or the OP screwed the Rally Armors to the tires rather than the fenders.

 

The only way I can finish these days is by jacking my rear diff.

 

Well I'm gonna figure what's wrong tomorrow. Taking my car to John Sines Import Auto.

 

My money is on a bent drive shaft, because you jacked up the car on the drive shaft, and bent the drive shaft. If the noise changes with the speed of the car, it is something that is rotating at the speed of the wheels/car, not the engine...aka wheels, transmission, driveshaft, etc. Also, are you sure you know the difference between the rear differential and the drive shaft?

Maybe you'll get lucky, and it's a stuck mudflap or something, but doubtful.

Regardless, if it is a bent drive shaft, don't be too hard on yourself. You're young, and we've all made plenty of dumb mistakes growing up (ask me how I know). Live and learn, and hopefully it isn't too hard on the wallet and it is an easy fix!

 

Well I don't mean to jinx anything, but...

 

So, uh, speaking of live and learn....

ALWAYS TORQUE YOUR LUG NUTS (in a star pattern!).

Top tip: as soon as you take a wheel off, wedge a paper towel in your door handle, throw something on your seat....anything to remind you to torque your lug nuts!

 

I used one of those automatic torque wrenches, not sure how the lugs fell off

 

Sick camber, brah.

 

Yep, always torque your nuts...I'm not sure just how that star pattern is supposed to work though. Torque them until you see stars?

Well, at least it didn't happen on the freeway. Great anti-theft idea though.

 

Stance nation?

 

Looks like you took out a couple studs to eh? Ive never had one come completly off but I have lost plenty of lug nuts by not checking mine regularly in my youth.

 

I check them with my star wrench once a week

 

5 speed right?

 

Auto.

 

5speed swap it, I've got almost everything you'll need. Can get the rest for you lol

 

I'd love to swap it, I just don't have the money to do it haha.

 

The first thing (or close to it) any young, aspiring home-mechanic should get is a torque wrench and learn how to use it and formulate a system to remember to tighten the lug nuts any time you have a wheel off the car. And...remember to re-torque them later if the wheels are alloys. Tightening them as hard as you can is not as bad as not tightening them at all, but that will cause you issues too. I waited to buy one until I effed up a couple of wheel studs and a number of lug nuts by deforming the threads. You are extremely lucky that this wheel coming off didn't kill you or someone else...and as much as we all bust your balls and tease you, we're all glad it wasn't any worse than it was.

 

On a MN car of that age, you also want to make sure there's no corrosion happening on the back face of the wheels as well. If you do have corrosion (such as the wheels on the Legacy I bought for my mom), the wheels will torque down nicely, but as the corrosion shifts/compresses, they can lose torque and then the wheel starts banging. This is part of why you're supposed to recheck the torque after ~50 miles.

Of the other hand, repeatedly checking the torque, especially without a torque wrench, will actually bump the torque up and up bit by bit each time you check it, and you can over-torque and cause the studs to snap (I know someone who did exactly this, or I'd swear it was made up).

 

Quoted $120 to fix it :/

 

Cheap lesson.

 

What lesson? Someone tried to kill me, TWICE.

 

You sure you didn't f up the first time, egg out the bolt holes while it was being retightened, and allow them to come loose again thus resulting in the above? If you think someone is trying to kill you, I'd suggest marking your lugs with China marker or similar so you can see if they've been spun.

 

Just buy 5 sets of the lug nuts that require keys, all different types.

 

They still have security guards in high school don't they?

 

Hahaha!

 

I had something similar happen a month or two ago but my wheel stayed put. Turns out someone had been loosening lugs in my apartment parking lot. I check mine about 2-3 times a week even after the kid was caught (and laid out by a ghetto mom in our building ROFL). As far as the $120 quote goes, do you have an air chisel and some basic hand tools? My Dad helped me with mine and we got away for around $25