Toroidal transformers are not completely symmetrical, because of the wire exits. The usual way of reducing hum induced by the asymmetry is to leave the leads long (or solder on temporary extension wires), loosen the chassis fixing and rotate the transformer to minimise or reduce below audibility the hum. Then tighten up the fixing bolt and tidy up the wiring.
Also make sure that the primary, and secondary wires are twisted. Open wire loops can induce hum and buzz in signal circuits through simple transformer action.