Nope...no concerns on this with the output transformers, just the PT.
Don, you should be able to change the diodes...those shouldn't be too hard to get to or change I don't think. They are on a tag strip right between the biggest cap and the one a little smaller. Same one as where Bob called the ground connection point as "Ground Zero".
I'm not sure how much diode change has of effecting xfrm buzz or not. But at the least it's a very good thing sonically anyway!
In the next couple days, I'll try to look up a couple of alternates that'd be decent quality and post them.
PS...I was actually referring to the screws that hold the transformer together, but the can over the transformer might get involved too! To get to the xfmr, you'd have to undo the screws from the bottom and left that cover off. You might listen at that point and see if the noise clears up even before tightning the screws. Also, check the four screws that mount the xfmr to the chassis. Heck, as far as that goes, you might tighten all the screws from the bottom side FIRST! Just in case....
Transformer buzz is actually a rather common problem and I think that I've seen where Hammond will not replace them when they do it new even.