A slight hum coming from my amp

Transformer. Many transformers have a slight hum, or develop a hum over time.

It is the 'plates' stamped out and stacked together to form the core of the transformer which vibrate slightly when AC current is running through the transformer.
Some transformmers have bolts running through them which you could tighten.
many do not.
You could place some shim, or device against the transformer in the right place to lessen the hum if it bothers you.
If it just a worry, then the noise is perfectly normal, and you can ignore it. No harm is done by the hum.

You could replace the transformer, but no gaurantee the new one will not him MORE.

Could also be DC on the line, which can be eliminated with a PS Audio Humbuster or a similar product from CI Audio.