Knocking/rattling from alternator housing, new alt didnt help
 

I have a rattling/knocking sound at idle coming from the alternator housing. If i take a stethoscope to the housing I can hear the rattle pretty good around the middle of the black shroud that the housing bolts into. As i move closer to the top of the shroud where it meets the housing it gets louder.

I had suspected the alternator bearing were going bad and put in a reman bosch alternator. Now the sond hasn't gone away. It might even be a bit louder.

I've got 6 shims on the fan pulley and tension looks good with 2 inside, 4 outside. Could the air duct that bolts onto the back of the alternator be making the noise?

Can you post a youtube video? I can't see the air duct making noise. Have you taken the fan belts off and run the car briefly to see if it goes away?

Will try disconnecting the fan as well as posting a video tomorrow. Thanks

If the noise is gone with the belt disconnected, I would suspect a cracked fan housing...

Just got home from work and took 3 videos

1st is just checking fan belt tension. Is this about right? 20120618_185902.mp4 - YouTube

2nd gets you a nice listen in on the rattling/knocking sound with the fan belt attached 20120618 190004 - YouTube

3rd is with the fan belt disconnected. Sound is lower but i can definitely still hear the rattle. 20120618 192112 - YouTube

A little more background on the knocking sound. When i took the alternator out over the weekend I found a decent sized crack in the alternator housing. I had it welded up by what i believe to be a competent magnesium welder. You can see pictures of the before cracks here. Unfortunately i didnt take an after picture before i out it back in.

http://forums.pelicanparts.com/porsche-911-technical-forum/683614-went-replace-alternator-housing-cracked.html

Sorry to resurrect an old thread, but did you ever figure out what this sound is? My car is making the exact same noise.

I noticed a little play in the fan pushing it forward (toward the front of the car). I'm also seeing some small cracks in the fan housing.

I had this noise recently from a worn woodruff key on the alternator/fan. New key solves the problem

Ok, cool, I'll check and see if that's an issue.

Sounds like it's time for me to park the car for a little bit and pull the fan stuff off to see what's going on, as well as get the fan shroud re-welded.

Check that the fan isn’t rubbing the housing.
The housings warp and crack and allow the fan to touch. It will make an awful noise

Just an update to my situation -- pulled the fan apart and found that I had a very cracked housing. Sent it to mb911, who welded it up nicely.

Just got everything back together and at least for now, the rattling sound is gone.

I rebuilt my alternator at the same time and found that the bearings were still in good shape, so I don't think that was part of the problem.

Thanks for the update. Nice to know the cause of the noise.