On batteries it works fine, but when i plug in a 9V plug it hums really loudly, and doesn't work.
Looking forward for solutions.
Thanks in advance

Correct polarity?
High quality, regulated, and isolated power supply?
'Fulltone' 9v power supply?

Did you buy it new? Could be a busted lead at the power supply input jack.

could be noisy power supply or bad filter cap in the pedal.

I am not sure what value electrolytic is in there but you could replace it and see if it's bad or if you still have the hum, but love the pedal, you could put a larger cap in it's place.

Most wahs use a about 100uf filter cap but you could go up to a 220uf or so.

Good luck

George