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Old 14th February 2009, 22:52   #1
gzig5
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Thumping Noise From Under the Car

I need help diagnosing a noise that started a couple days ago and continued to get louder until I thought something was going to fall off! Over three days I drove back and forth to work which is like six miles round trip. At first I thought I had a small exaust leak, until I realized the noise changed speed with the car, not the engine speed. Almost sounds like something wrapped around the driveshaft and is slapping the bottom. Seems to be coming from right under the seats. I put the car up on ramps (still not a lot of room under there) and I can't see anything out of the ordinary. The drive shaft seems tight, and that bushing/joint where it bolts to the transmission looks OK as well. Wheels lugs are tight at each corner and I couldn't see any evidence of a broken mount. The frequency of the noise definitely increases or decreases with forward speed, so I suspect a rotating component, but I'll be damned if I can find it.

Any ideas? Clutch, transmission? I haven't got a look at the rear end yet, but that is next if I can jack it up.

Greg
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Old 14th February 2009, 23:26   #2
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Guibo?
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Old 14th February 2009, 23:27   #3
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What the hell is a Guibo?
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Old 14th February 2009, 23:29   #4
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Can you clarify what you inspected? Was it 2, 7, or 16 here:
RealOEM.COM * BMW E39 M5 DRIVE SHAFT-CEN.BEARING-CONST.VEL.JOINT
Any vibration to speak of? What if you unload the transmission at speed, say coast in N?
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Old 14th February 2009, 23:32   #5
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yep it is the guibo. Its a rubber coupling in the middle of the driveshaft. BMW refers to it as a flex-joint.
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Old 14th February 2009, 23:48   #6
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Chowdah,
All I could really see is part# 2 at the front of the driveshaft, back of the tranny. It looked OK, not all cracked like a picture I saw on here recently. I couldn't feel any excess play. If I unload (push in clutch) or am not on the gas the noise is present, but less noticable. It is very apparent while accelerating or holding speed with the throttle. No vibration other than what the noise makes resonating through the car.

I can't get the car very high because my jack is a POS so I'm limited what I can see towards the rear until I can borrow a better jack. That plus the fact that the car is in the driveway and it is 28 degrees out and laying under the car is a bit tight with a winter coat on, my inspection options are limited. I'll try to get to that center support. I assume that is about halfway to the rearend from the tranny? I'm new to this car if you couldn't tell already.

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Old 14th February 2009, 23:58   #7
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You inspected the guibo (#2). Your research has covered the bases, which is impressive as a new member (welcome to the board btw). A person recently mentioned replacing #7, which may be informative.

If the ground out there isn't slippery, you may want to get some ~$40 Rhino Ramps as a complement to a jack. Effortless compared to jacking, but not always a substitute.
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Old 15th February 2009, 00:52   #8
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All I have is ramps right now. At least it is dry. I backed it up the ramps and everything looks good in the rear. All the bushings are in one piece. I took it for another driven and the pitch changes when you let off the gas and engine brake. Sounds just like a bad exaust leak. Doesn't matter what gear you are in, it is definitely related to drive shaft speed. I went as fast as I could in reverse and did not hear it until I let off the gas and engine braked and then it got loud. What the heck does that mean?
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Old 15th February 2009, 02:05   #9
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The fact that you've mentioned transmission, but not vibration, coupled with the fact that you've mentioned an exhaust sound twice, means you may wish to read up here on "blatty" exhaust sounds and "CPS". "Thumping" isn't a word I've seen associated with that problem, but maybe that's it.

As a new owner, you'll want to invest in a Peake reader, because one day, your car will throw a Service Engine Soon light, and you'll want to know what's going on w/o going to the dealer. That goes doubly if you are handy.
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Old 15th February 2009, 03:10   #10
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It has nothing to do with the engine. It will spin up smoothly to 6000+rpm. It only occures when the car is moving and it sounds like the exaust leak I had once on my 72 "Cuda. But it is not an exaust leak. The noise goes away for an instant when I am between gears. I'm not sure if that is because the tranny is the source or that the driveline is unloaded at that instant.
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