Whistle sound in gear at low speeds

When I shift from park to drive or reverse the noise goes from intermittent to consistent and louder. Noise is non existent when cold, doesn't take long to start maybe 5 min. It sounds like a whistle, not air flowing out of something, like the shrill sound of a whistle. It sounds like a bad bearing on the belt, just don't know why the sound gets worse in gear. I drive a 525i with steptronic transmission. I post this as I am suspicious of the transmission.

Sounds like a hole somewhere along the intake

It sounds like a grinding though, not airflow. a whistle sound is just the best way to describe it, high pitched and shrill. I'll take a look at the intake. I did recently have the box out and fan off when I redid the ofh gasket. Also did the whole cooling system minus the water pump as it looked to be replaced earlier.

I'm going for the fan clutch first I think. After further inspection sounds like its coming for the serpentine belt for sure . Perhaps the timing chain... Again doesn't make since with the gear change, rpm doesn't change the slightest.

Perhaps the bearings in one of the serp belt idler pulleys. You need a mechanics stethoscope to diagnose, or take a piece of hose, put one end in an ear and the other end against where the pulley is connected. You should be able to tell which pulley is bad (or an alternator bearing).

If you still can't tell, take the serp belt off and spin the pulleys. If one makes a rough noise and doen't spin freely, you've found the culprit.

BTW-fan clutches do not usually fail in this way.

Ok, I'm 90% sure it's my alternator or the tensioned nearby. Can someone please give me the slightest explanation as to why it makes the sound in gear and not very much in park? It will make me feel better about getting into it. I'm afraid ill be wasting my time taking the belt off to check the pulleys.

Well they all spin freely, the tensioner pulley makes a bit of noise when I spin it so that's where I'm starting. Bought a stethoscope and it didn't help. With it touching the ofh I could hear the noise so I guess the tensioner makes the most since. I'll confirm when it is solved

It was the water pump adjustable belt tensioner, mechanical style. not the bearing, but the spring inside. Still don't quite understand the difference in sound when in gear, but for those of you with my problem seeking problems that are pertinent to the shifting, perhaps the tensioner is a good place to look. Just hop under and tug on the belt to make the tensioner move. If it sounds creaky, most likely your issue. Thanks everyone for the help

Thank you skibby very much for a thorough documentation of the problem, symptoms and the solution to the problem. These threads are gold when someone is searching google.