Well, I don't think I'll be able to help you much.
If you can't hear the squealling when you're driving downhill or uphill, but only going on level ground, I don't have a clue what it could be.
One idea though........the rear exhaust mount has been known to cause squeaks. If it were my car, I'd get under the car & spray the mount with some WD40 (or anything similar), just to temporarily see if the squeak stops.
Other than that.........
2000 740iS. MKIV, 16x9, DDEs controlled by Euro fog light switch, reversed steptronic +/- pattern, paddle shifters, clear corners, Basslink, iPod video with stereo audio, TV tuner, gauge rings, ///M pedals, backup camera with 'on demand' switch, quad brake lights, Dinan engine & tranny software, windows/sunroof close via remote.
2003 540iS. MKIV, 16x9, PDC, CWP, TV tuner, backup camera with 'on demand' switch, paddle shifters, windows/sunroof close via remote, iPod video & audio.