I think I've gone through pretty much all of the problems you can have with the rear end of the car that will make distinctive noises, so I'm going to lay some of them out for anyone who has problems in the future.
I'll start with the most severe:
My car sounds like...
Pull over now!!!
The most probable cause of the sound is one or several loose lugs. Stop the car, bust out the BMW tool kit (the tire iron is actually over on the battery cover) and tighten all of the offending lugs. While you're at it, go ahead and check all of the rest of the lugs too. If the lugs work their way out far enough, the sound will get worse and the shaking will pick up. Eventually the lugs will snap and you'll note you'll be riding a bit lower on that side. By then, it's generally a bit too late - your car's on the ground.
...it has metal scraping around in back:
It might be that the rear shock mount (RSM) has failed. This failure, however might not have occurred at the shock mount itself, unfortunately. The metal scraping and knocking could mean the failure occurred at the frame mount. This is going to require some metal work to fix. Plan on spending a good amount on fixing it.
To prevent this, upgrade your shock mounts and re-enforce
them! Re-enforcement plates are available at BavAuto, among other BMW parts shops.
...there's a slight knocking or squeak in back:
As mentioned above, there is probably a problem with the RSM. A knock means that the RSM has probably failed and the upper mount of the shock is just knocking around freely. Squeaking generally only occurs when the roads are wet, but can occur at other times, and generally means the same thing. Both the knocking and squeaking should co-incide with suspension actions, such as driving over bumps, or rough roads. This problem isn't as urgent as the above-mentioned, but since the mount has failed, it's possible that moisture can now get into the car, which can cause other problems. Get new rear shock mounts. Meyle makes a heavy-duty RSM that resists push through failures (most common type), yet retains the stock feel, unlike many of the urethane upgrades. If you're going performance, now is a good time to upgrade to urethane. When you replace the shock mounts, don't forget to buy a re-enforcement plate! Replacement RSMs are available at most all BMW part shops, and re-enforcement plates can be found at BavAuto, and maybe Pelican.
...it's running on off-road tires:
This is most likely a failure of the rear wheel bearing (RWB). The rear wheel bearing failure's sound can range from a grumble, to a growl, to a roar. When it gets bad, it will sound like you're riding in a Jeep with huge off-road tires
. The sound will follow your car's speed, not the engine RPM. Generally, as you get faster, the sound gets louder. This is not always the case, however, as sometimes there is a point at which the sound will die down some. To check the speed correlation, bring the car up to highway speed and press the clutch pedal (if you're an AT, put the car in N - this is illegal in many states, so watch yourself
) to let the engine spool down to idle, while your speed remains relatively high. If the sound remains strong, it's most likely the bearing. If the sound dies with the engine RPM, you've got something else wrong.
RWB fixes are nasty! They aren't super-urgent, but be aware that if the bearing finally fails completely and locks, you're not going to be getting anywhere very quickly and you'll probably need to replace the tire that just got dragged until you stopped. I drove on mine for a while before I replaced it; some 4,000mi, I think. That's not the smartest option, but I needed to save up for the parts and tool.
This fix is time-consuming and expensive! The tool to remove and press the bearings is around $250 itself. The bearings are around $30 each. If you have a while (and the patience!!) to do this job, this is your best option. You can do both bearings on the rear axle for the price of what most garages charge for one. Expect hang-ups if your car has not had much service in the rear. Frozen bolts, hubs and rotors all slow progress and are a general pain. Once you get it done, however, your car will sound like new again. The tool and parts are available at Pelican Parts (they have the best prices on this job, believe you me!).
You will need:
2 RWBs, 2 Circlips, 2 New Rotor Screws (one-use part, don't re-use the old one!) and the bearing press. A Bentley Manual is an absolute must! If you don't have one, PM me and I'll send you my write-up with alternatives, should one of the options not work.
This fix is expensive! Expect the shop to charge around $300-350 (that's at independent shops). It will be a bit less on the second bearing if you're getting both done. BE AWARE
that some BMW specialty shops may try and screw you. One BMW shop that I trusted quoted me at 7 hours of labour, plus over-priced parts, so watch yourself. I could've gotten the job done in 1 hour a side if I had a lift and shop tools. Seven is about what it took me in the end with socket wrenches, jack stands and a rubber mallet.
First, check and make sure you don't have a hole in your tires
. If that's not the case, and it sounds more sandpaper-ish than high-pressure air, then it's probably something rubbing. More specifically, the brake rotor shields. If it's very light-sounding, it's probably fine. If it's more gouging or metallic-sounding, you should probably try to re-bend the shield so it's not rubbing so intensely. The best bet, however is just getting them replaced. The shields themselves are not too expensive, but the job is similar to the rear wheel bearing job, as the rotor will need to be removed and the hub will need to be pressed off to get to the brake shield. I haven't gotten a quote from a shop, but I'd expect labour to be the main cost in this fix.
I hope this clears up some of your sound worries. Good luck with the fixes!