Under it ??? What you mean ??
ohh, I forgot to mention that my skimmer is inside the aquarium
Well that changes things......
Pull the airstone off the bottom of the skimmer slightly so it can't buzz against the bottom of the skimmer. If that doesn't work, I would suggest you get used to the noise or buy another skimmer, none are really noted for being silent, but some are noisier than others