2003 Ford F-250 Growling sound when in motion

I am hearing a growling sound coming from the left front wheel, I think. I can only hear it when I am moving slowly and when I let off the gas. Somebody told me I might need the wheel bearings repacked. I took it to a mechanic and it is the kind that cannot be repacked. I have to replace the whole hub. Costing about $500. I would like to get a second opinion before I do it. He had it up on the rack and spun that wheel and heard nothing. Can it be the weight of the vehicle makes the bad bearings growl? Can anybody tell me what the problem might be?