[Are]

The pulleys, tensioners, water pump, and belts have been recently renewed. From cold the car starts quietly enough, but during the warm up period a loud engine speed noise develops in the alternator area, and then after a few minutes the noise fades and the engine runs quietly!
The noise is hard to describe, it is not sharply metallic, but more of rubbing sound it is only audible when starting the engine from absolutely cold and will not occur again during daytime after the car has been used. By hand pulleys and pump turn with normal resistance, and different belts have been tried, the alternator turns freely but without noise. A BMW garage could not detect the noise.
I should be most grateful for any help or advice. Thanks Are

 

[mcbonio]

Is your engine fitted with a secondary air pump?

 

[Are]

mcbonio, thanks for your response. I don't know where I should look for a secondary air pump. Would you advise please?

 

[mcbonio]

See the round component on the left with an angled flat surface on it? Some M54 engines were fitted with these and on a cold start they can make a right racket when they start to leak air. Does yours have one?

 

[Are]

Thanks mcbonio with your input I was able to eliminate the secondary air pump. The noise is getting worse and it is directly from the alternator, perhaps the new belt and tensioner put additional strain on already failing bearings, but I still do not understand why the engine starts quietly and then the alternator begins to rattle and then becomes quiet! Thanks again, Are

 

[mcbonio]

You need to make a mechanics stethoscope, long metal rod you can put on the alternator housing and to your ear to pin point the sound. It may be the alternator bearing as these are well known to fail. Bearings are funny things that can make all kinds of noises depending on conditions. Hot, cold, under load, off load, speed ect.

My water pump bearing went and only screeched when it was hot and revved over 4k rpm which then would screech constantly at idle. Turning the engine off then on again would make it stop, strange!

 

[Are]

Yes thanks I used a screwdriver stethoscope, and it really is the alternator, it sounds as rough as can be. Are

 

[Bouncypete]

Some cars have a sprag clutch in the pulley on the alternator. I'm not sure if the e46 does but it would be worth popping the belts off to have a look. You can also squirt (carefully) a bit of WD40 into the pulley whilst it's running AND making the noise. Don't aim the WD40 on the belt, aim it in the center of the pulley. If the noise immediately goes away then you've identified your problem.

A Sprag clutch allows the alternator to freewheel when the engine is decelerating and works like a dual mass flywheel smoothing out the power pulses at idle. (click the image it's animated).



NOTE - you do not need to replace the entire alternator if the pulley is faulty. GSF are likely to sell the pulley but you'll need a puller to get the old one off. It will be easier to find a company near to you that overhauls alternators and they can change the pulley for you.

 

[Are]

The alternator is off the car, it turns quietly! but with a slight tick, tick when turning it back and forward, uhm, the fact that the loud noise occurs during the warm up but only lasts for a few minutes and now sounds mechanical, seems to support Bouncypete's "Sprag clutch theory", (nice animation), there is a sense of vibration and rattle when listening with a stethoscope on the alternator body, and the noise is clearly audible outside the car with the bonnet down.
There appears no space in the pulley where a sprag clutch would be located.
(The only sprag clutches I knew of were on the freewheel Saab 96, and I think that they had spring loaded ball bearings.)
The alternator is a Valeo 120, 14v, 70/1204 (does BMW fit Valeo Alternators as original equipment?)
I think that round here taking it to a specialist repair shop would be too expensive. Thanks, Are

 

[Are]

The noise was caused by a stone, some leaves, nut shells, and insects that had been drawn into the alternator! Great! Thanks to mcbonio and Bouncypete for their responses. Are

 

[mcbonio]

Brilliant work, nice cheap fix then.

 

[dailydoggy]

valeo are OEM along with bosch