Could be the clutch release bearing. Does it go away if you put a little pressure on the clutch without depressing the clutch pedal (load up the clutch release bearing without letting the clutch slip).
Otherwise it's a gearbox bearing. If it does it in every gear than it could be any of the bearings on the main or lay-shaft.
When you changed the oil, did any metal come out with it? What oil does it have in it? it should have dexIII ATF with the lubrizol additive and not a regular gearbox oil as it's to thick and that too can cause some noise as well as poor shifting.