I wouldn't have said it if I wasn't sure. Remove the EL34s one at a time, if one is dodgy the noise will stop when it's removed.
If the noise doesn't stop when you've ruled out all the EL34s, move on to the ECC83s starting with V1 (the one closest to the input).
Have you got any spare ECC83s? - if so try swapping the ones in your Marshall one at a time to identify the dodgy one.
:twisted: