I experienced this with one of mine once. Constant low-level hum, didn’t get any louder with volume and so obviously didn’t cause an issue at gigs/rehearsals. Still it used to get on my nerves.
I took it to a tech and it turned out to be the position of the transformer which is obviously secured with a bolt to the bottom of the casing. It must have turned slightly during its long life. After a bit of trial and error he rotated it slightly then tightened it back up and hey presto, the hum was gone.