Tube Ampp Almarro low level hum

My Almarro 318B amp has recently developed a low level hum that comes and goes. I have swapped out all 4 tubes at one point or another and that does not change it. Once the hum stopped for a few days while I was breaking in a new paid (NOS) of 6C33C tubes. But it keeps coming and going. It gets a bit louder and then softer. If I tap the base of the amp, the hum stops. If I pick up the amp, it stops. If I knock the table where the amp sits, it stops.

I run the amp about 6-8 hours a day. Once a week I foget and leave it on all night.

Any ideas?

Thanks

Dan