Can anyone help me diagnose this problem with my 84 Westy?  I am getting a fairly loud low pitched whiring/whining sound from the transmission.  I thought for a while it was just the sound of my new tires purchased  about 6K miles ago (it sounds just like knobby tires on pavement) but now I don't think so.  The noise does not change with road surface and is just as loud when driving on fresh snow.  The sound is the same regardless of gear selection and is speed dependent (wheel speed not engine).  It is most noticeable at about 10mph. The noise does not change when cornering.  I am wondering if it could be differential or CV but I do know what these sound like when they start to wear.  Any suggestions?
Many Thanks
David

BTW: I was asking for information from the list about engine replacement/repair a couple of months ago.  I ended up getting my engine rebuilt by a local (Portland, OR) mechanic for $2200.  So far (about 1k miles) so good.  I decided not to go the rebuilt engine route because of quality concerns (GEX etc.), and because this was about the same cost and included the labour (the van is my primary means of transport so I needed a quick repair).