Hiss with AC-DC buck supply

Hi everybody,

i bought two bucks 220V to 9V like this

https://www.ebay.com/itm/NEW-High-G...hash=item3aedf56b33:m:mU9l2J_hWAfiMR8zs66GTXw

to power two pedal effects for my guitar. It works all but I hear from the speaker a hiss that changes the frequency depending on the changes I make to pedal effects. I would like to be able to fully delete this hiss but I do not know how to do it.
Can someone help me to fix it ?

Many thanks !

I add that a linear power supply with a transformer I have already realized but, in addition to being a bit bulky, that would not be the real problem, it interferes with the inductance of the wah wah I have in the pedalboard. In this video
a mu-metal sheet solves the problem of noise, but it costs too much.
Cheers !