200SX Whistling (like a tea kettle!) while driving

[98NISSANGUY]

Hopefully someone has come across this problem before as it has me stumped! I have a 1998 Nissan 200SX (33,150 miles) that has suddenly developed a LOUD whistling noise while the car is moving. As soon as I stop (stop light, sign, etc..) it completely goes away. The second I take off... it starts whistling again. Its a non-SE, SE-R (has base engine) and has 33,150 original miles. The whistling noise itself is constant (doesn't seem to increase/decrease in volume if I am slowing down or speeding up) and it doesn't happen all the time (short trips, etc... usually doesn't cause the noise to start) It usually happens after i've driven the car at steady highway speeds (65+). I've checked the fluids, belts, coolant and everything seems fine. Any ideas?!?
Modified by 98NISSANGUY at 7:11 PM 5/13/2006

[180fan]

is this coming from the engine bay or is it being produced from the speakers?

[98NISSANGUY]

Its definitely coming from the engine bay/front end.... definitely turns peoples heads when its occuring too. I've driven it yesterday and today... not a peep of noise but I haven't driven it very far (to/from work) but did have it up above around 75/80mph.