Assuming it's a linear power supply then it's most likely that the laminations on the mains transformer are vibrating - this shouldn't happen. A linear power supply has a standard mains transformer.

If it's a switch-mode power supply (lots of electronics in the box, small high frequency transformer) then it's a different matter.

Does the hum / buzz stop when it's unplugged from the pedal? If so then it could be that the power supply is trying to work too hard.

What is the current output of the power supply - and what is the current requirement of the pedal?

Where are you located?

John