all transformers humm, some very much so. Usually they are bolted in tightly, or have some elastomer in the bolt to quiet the humm.
You may have an issue where the transformer has loosed up, and is being allowed to vibrate. If it has elastomer it might have been squished out, deformed, or come apart.
Pull the amp out, disconnect from everything, pull the case, and see if you can move the transformer around. Also check to see if anything holding it down is loose. If it has elastomer, it should be "tight" but not so much so that the elastomer disformed too much, or gets pushed out.
It's like a 10min fix, also normal.