I would imagine that something is just rattling.. I say this because if it was an issue of possibly a pulley or a wheel bearing or anything at all for that matter. Then the noise would be more consistent. It isn't too often where you have a vehicle that only has an issue in that tight of a situation.
mostly when traveling steady at 35, 40 or 45 miles per hour. Even 1-2 mile per hour difference will cause the noise to stop.
The noise will go away if I accelerate slightly or slow down by letting off the gas. I've noticed that it will also make the same sound around 20 miles an hour,
This leads me to believe that it's an issue of something vibrating. At a stead 35 to 45 miles per hour the engine should be I'd say about 2500rpm on the highway. Around the same rpm that it would be at when coasting 20 miles per hour. A slight increase in speed also is directly related to a slight increase in RPM, and the noise is gone. Turning, (lets build a scenario of you turning right at a read light) you touch the gas slightly and turn. Most likely hitting right around that 2300-2500rpm mark to get the noise to occur.
Next time you're in the vehicle and you hear the noise take note to where in the RPM range the motor is at at the given time. Then when you get back to your parking spot try to replicate the noise while stopped. If you cannot get it to happen while stopped then perform the following test.
Fully engage the parking brake, place the vehicle in drive, and place your left food on the brake pedal. Then with your right foot start to bring the RPM level up (use common sense here, don't just put a brick on the gas pedal because you'll end up slamming into whatever it is in front of you).
Using the second method if the first one fails will ensure that all the components are engaged and being torqued as if you were driving the vehicle down the road. You'll feel the entire car lurch up (resembling a cat ready to pounce).
If you can get the noise to reoccur at will then you can have someone assist you in pinpointing it's location.