I bought this used Caprio 1999

I bought this used Caprio 1999. Everything was working smooth. After only driving it for 3/4 of a tank there suddenly was this loud metallic bang underneath the car somewhere. Immediately after, a rattling noise started every time the gas pedal was not engaged. The rattling noise disappears as soon as I put my food on the gas pedal. The noise is only there when driving. I took it to the mechanic and they said there was a leak in the trans axle seal and there was no oil in the trans mission. They put oil in and said I would probably need a new transmission. It runs well, beside from the rattling noise. Gear shift is smooth, motor sounds good and the oil is not leaking fast as I have checked it two times after and it was still full. My questions are. Could it be something else than the transmission? If it is the transmission, which part of the transmission is it and do I need a new one or can the old one be fixed at a lower cost than getting a new ( used one). My plan so far is to drive the car and check on the transmission oil regularly and mean while look for a used transmission. Which model of transmission should I look for? Can it be from other VW models and which years will fit?If it is the transmission that is damaged Is there any risk of damaging other parts of the car driving it? Hope for some clarity. Thank you and Aloha. Nanina