Then;
Start the engine. As soon as the engine starts, the entire clutch begins to turn, including the input gear and cluster gear, but NOT the TO bearing, because it is NOT touching anything, you made sure of that. This is irrespective of if the trans is in Neutral or not. If you have this noise in neutral, with the rear wheels Not turning, the only thing that can cause it is the front bearing.
To me, it sounds like the trans has no oil in it. If true, and the car was operated like this, then the trans may have been toasted.
Test-2
Check the oil level and fill it as may be necessary, then go for a roadtest. Get the car up to top gear. If the noise more or less goes away, then the diagnoses is correct.
In top gear, everything inside the trans on the mainshaft from crank to driveshaft is all spinning together at the same rpm. The cluster is going for a free ride, and there is close to zero power being pumped into it; and so the front bearing is doing little to no work other than just supporting the input gear. In this mode, the noise should be considerably quieter.
