Strange sound coming from rear wheel?

[cdcd]

I have had this problem for a long time now and cannot pinpoint where the sound is coming from. It sounds like an ambulance siren only not as loud. It only makes the sound after I have driven it maybe 10 miles, like it has to get warmed up first. Could it be an axle bearing? Do they go bad often? It's a posi rearend if that helps. Please help me with this. It is embarrasing.

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72 c-10 lwb 350/4speed/3.73 posi