Piaggio vespa GT_ knocking from transmission case and other issues..

Scoot is new to me ,. It had been sitting for a few years in a shed, got it started and fired up immediately now some issues.......

I heard some occasional knocking from the transmission case area when on indle and on centre stand.

On the over run I sometimes get backfire

When on stand and I twist throttle, throttle snaps shut but sometimes takes a while for revs to settle down.

Found a drain pipe but nowhere to connect it...

Any ideas on the above issues??