Get rumbling noise when coasting?

Whenever I take foot of the gas and coast, I get a rumbling, vibration type of sound from I assume the rear end somewhere. As long as the wheels have power going to them there is no noise. Noise only lasts for about a five mph drop in speed or so then is quiet again. Just wondering if anyone else has has this issue and where/what to look for? Guy at work said to put Lucas treatment in the differential??

Update: Topped up fluid level and now the sound is just a brief vibration when I let off the gas at any speed, like something is worn or loose. Once I have the funds will take plate off and see what there is to see.