1992 Chevy S-10 Gears making noise

When driving and shifting the gears are making a growling type of noise. At times will do this while idle. Was told to check and add oil to rear end. Hubby swears it is rear end going out. See no place to add oil to rear end. Vehicle runs fairly well for all the miles and has had very few issues till this. It is kind of like the noise a manual tranny makes when you do not shift properly only while shifting properly. Any Ideas?