Hello Jonnoh,
Is the noise persistent with the volume control all the way up and all the way down?
You could try swapping in a different set of tubes if you like (12AU7 and 6080). It may also be enough to pop each one out and reinsert it back in the socket, just to ensure good contact.
It would also be good to look over the voltages to see if anything is drifting, as these may indicate a tube that is being fussy.
-PB