E39 speed-related noise

My 80,000-mile 1999 523 auto has been making a speed-related wah-wah-wah sound that's been slowly developing over the last year or so. I've had it
checked by a garage I trust and there's apparently nothing wrong with the wheel bearings, brakes, shafts or any of the other oily bits underneath that go round. The tyres too have been checked - no flat spots or bulges etc.
Since I've had the car from new the tyres have been replaced three or four times and the brake pads once, but no other wear-and-tear items have been replaced.
The noise isn't harsh, or screeching or grinding - I've listened at the side of the road as the car has been driven past in neutral at 20-25 mph and I can't make out where the sound is coming from.
Any ideas?
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"eNautilus" wrote

How old are the tires? Some tires, when they wear down, have a tendency to moan/howl.
Pete
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Check rear seat -- Mother in Law might be lurking?
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