Agree that this sounds like transformer hum. Hopefully from the external sources as this would be fatally expensive to replace any of the 3 xformers in your amp. Try to eliminate other sources as mentioned above.
ARC REF 150 Buzzing noise
I have an ARC REF 150 amp with over 4000 hours on it, and recently it has developed a buzzing noise. This appears after about 30 minutes of use, and seems to get steadily louder. I thought it might be a tube. I had KT150 power tubes in it and I swapped those out for KT120's and re-biased. No change. I then swapped out the 6H30 input tubes and re-biased again. Buzzing is still there. It is very difficult to locate the sound on the boards. I have also had several blown fuses, this has been in the midst of a move and may or may not be reIated to the buzzing. I have to assume this needs a repair, but I am wondering if anyone has a guess or even better, experience with the problem. I am in the Denver area and if anyone knows of a local repair opportunity, I would appreciate knowing that too. I do not look forward to the expense and delay of sending it back to ARC repair. Thanks. Scott