Phono Stage - Transformer interference

I suspect my transformer is causing interference with my phono stage. I'm getting a buzz (?) in one channel (nothing connected). After trying everything else, I turned the board in relation to the transformer & moved it further away. I can make the buzz disappear or change channels...

- The transformer is 18-0-18 266VA, I realize rather oversized but it was all I could get off the shelf in CT at the time
- The noise is out of listening levels.. from approx 11 O'clock on is irritating, but but not discernable above the music, as after 11 is getting damm loud  . But anyway I must fix.
- Phono inputs are approx 5cm twisted, output 15cm twisted, all not shielded
- I have checked for bad connections dry joints etc, pretty sure this is not an issue.
- Transformer is as far away as possible
- No noise on the linestage
- No noise when phono is running as MM, only when I switch to MC.

So what to do..
- Will a smaller transformer cause less trouble?
- Should I look for a shielded transformer?
- I assume shielding input/output cables may not be the answer, as the tracks on the board will probably still be impacted?

Any input will be appreciated!