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Anyone heard this noise?

My '96 328i has developed a strange noise that has me and my dealer
mystified. I'm hoping someone else might have come across this and knows
the solution.
	Somewhere around 115-140 km/hr my car  often starts whistling. It goes
on for 5-30 seconds and then subsides. To make matters worse, the sound
is intermittent.
   It sounds like a loud wind whistle, but since it's coming from the
middle or passenger areas of the car I can't pinpoint it while driving.
Of course, when I took it to the dealer today, it didn't happen, even
though the noise was there 10 minutes earlier. Although the car is three
months old, the problem didn't start until three days ago.
   Oddly, it happens after the car has been sitting or driving in the
hot sun, and only after the car has been stopped for a while and then
quickly accelarates (for example, the on-ramp on a highway). It happens
with A/C on or off, with the circulation fan on or off and with the
recirculating feature on or off. It isn't likely to be a window seal
because I have experimented with opening and closing each window and the
sunroof while the noise happens. The dealer adjusted the angle of the
windshield wipers just in case that was the problem and checked the door
seals. It's still happening.
   Last night the car was in my garage, and first thing this morning it
didn't happen. So I decided to experiment and leave the car in the hot
sun for an hour--back on the highway and there it is again. I repeated
this procedure a few times during the day, and it is pretty much
guaranteed to cause the problem. However, as usual it fades away and
disappears. It will go away if I slow down, but not immediately, which
seems to indicate it has nothing to do with engine speed but something
to do with the speed of the car. But once it's gone I can resume travel
at the same speed that caused the noise seconds earlier and there will
be no sound.
   I think it may have to do with the effect the sun on the outside temp
of the car. Here is Toronto the past few days have been the hottest
since I bought the car, which offers another clue. I first noticed the
problem Saturday on a highway on-ramp after the car sat in a mall
parking lot for an hour. But it has also happened after stopping briefly
to refuel at an expressway filling station and driving back onto the
highway on the entrance ramp.
	If it was regular wind noise I would expect to duplicate it easily at
the same speeds, and wouldn't expect it to go away after a bit of
driving. Because it does fade away, the problem has subsided by the time
I get to the dealer, about 15 minutes away.
  Has anyone come across this one before?