The only time I had a really noisy transformer in something that just would not shut up by messing with the bolts, I loosened the screws on it and brushed a bit of thinned varnish on the lam stacks. I let that run in for a couple minutes and then tightened it up. It was quiet after that. I think the varnish that originally was supposed to be keeping the lam stacks tight had just cracked up over the years and they were buzzing against one another.
When transformers go bad, they get hot and start stinking and dripping goo. If yours isn't doing anything unusual, I wouldn't worry about it.