BMW Repair: e36 howling noise, wheel bearings, rear end

Question
howling noise sounds like it comes from rear end when im low on gas. what could this be?
thanks

Answer
Only two thing I can think of that would happen when low on gas is less weight.

If it's repeatedly coming when you're low on gas, wait until it's there, then add significant weight to the trunk.  If it goes away, it's weight related.

That could be anything from suspension to wheel bearings.  But it will be much easier to diagnosis at that point.