1998 540i squeals like air being let out of a balloon

[DonnaTate]

My 1998 540i makes a strange sound, usually starting once the car reaches 30mph. It's a high pitched squeal, sort of like letting air out of a balloon.
It's coming from the front end somewhere (sort of sounds like its underneath the car), though difficult to pinpoint because it doesn't make the noise while idle.
Any thoughts?

[brettski]

Welcome to the Werkz

Does the pitch of the sound vary with the revs or the speed of the vehicle?
Will it do it when you are stationary?

[DonnaTate]

It gets a bit louder as the speed increases. It also seems to slow while braking. It doesn't make the noise while stationary, and the only time I've heard it below 25 or 30mph is while braking to turn a corner etc.

[brettski]

Check your brake discs, there is an anti rattle clip that can come into contact with the rotor if the pads are really worn down.
Has there been any work done on the car recently?

[Penngwyne]

I had this problem on sons 525i (except for the braking bit which makes it different)

Put duct tape from the windscreen over the top of the grill below the wipers.
The grill vibrates and makes horrible noise which gets worse in a headwind.
The noise was really loud too.
Permanent solution would be to pack the grill with some rubber strips maybe?

If that doesn't work maybe what Brett said or check wheel bearings?