Mine as well.
I have had this happen on a few pieces of equipment. The cause each time was a transformer mounting bolt which had come a little loose over years of use. Gently tightening the bolt resolved the issue.
Transformers naturally vibrate a bit. It is in their nature to sing along.
P.S. The issue to which @Rudolf_Appel refers impacted only a few amps of very early production and was resolved long ago. Subsequently manufactured amps do not have the issue. I suggest contacting P.S. Audio describing the problem and providing the serial number. If you would like, you can email Paul directly at Paul@psaudio.com.
P.P.S. Only the first two months of production were potentially impacted. Any amps built in August 2015 forward are not affected.
If your amp is six months old you are fine. But to read your serial number, here is an example - serial number 5G0032. The first character is the year, 2015. The G corresponds to the month, in alphabetical order. G is the 7th month (counting from A), and then the last two are the unit ID. So, this serial is number 32, built in July of 2015.