What is causing and possible fix for noises when coasting?

The noise, which my boyfriend feels is a mechanical not a tire sound, happens when coasting. There is a whine that happens when driving and gassing the engine, but the louder noise is when coasting. I have a tire that loses air and this is when the noise is loudest. However, it seems to come more from the front side than from the driver's rear (which is the tire). I had one guy say transmission and another suggest the transfer case. My transmission seems fine though.