Toyota Repair: toyota sienna 1999, differential fluid, axle bearings

Question
The Van has 102000Km 1999 Toyota sienna. A noise has started. the local garage seems to think it is in the transmission(may be a bearing in the differential). The sound is like a whirring/scraping it is apparent at low speeds and at highway speeds sounds like a turbo.
It does not occur when in park or neutral while the engine is revving. noise is greater on acceleration and is noticeably lesser when your foot is off the gas.
there is no difference to the pitch of the noise when turning
left or right. There also is no change to the pitch of the noise when the transmission shifts  

Answer
It sounds to me that the noise is in the differential, it's not that difficult to tell which one it is, have them raise the van up on the rack and put it in gear, I doubt that the transmission is the problem because it probably wouldn't shift correctly with all that mechanical noise. It may also be a problem with one of the axle bearings, this is very common, it will be obvious where the noise is coming from with the van in gear and the engine running. I would also recommend draining the differential fluid to see if there are any metal pieces in the oil and on the drain plug magnet.