1999 Jimmy noise

2 days ago while highway driving(60 mph) my 199 Jimmy sls started emitting a noise almost like a moan or roar. At first I thought an 18 wheeler had pulled up behind me. The noise increased in pitch and volume until I pulled off to the shoulder and turned off the ignition. After re-starting , the noise hasn't come back.
If any one has an idea on what might be the source of the problem, please reply.