here are some thoughts, I would double over all nuts and bolts in the general area, could possibly be trans mount cross member bolts. I would definitely run it with the axle tubes on jackstands and wheel chocks in the front and back of the front tires, WITH THE CAR POINTING OUT OF THE GARAGE with nothing in front of it, to see if noise is there with no road vibration. When the noise is present You might want to step on the clutch and turn the ignition off to see if noise is still there with engine off. If you decide to take the speedo gear out you could probably jack the back of the car as high as you can get it to minimize loss of oil. While in there you should be able to see / poke at the vent baffle tin that is in there to see if its loose.
