This is typically caused by water in the airline and would be related to it being too deeply submerged at present or at some point. There are two ways to resolve this.
The quick way which will help but not eliminate the noise immediately is to unplug the skimmer, close the airscrew completely, make sure the marked water line is above the actual water line, regard this line as "maximum" and the minimum is about 1" below that, the ideal is 1/4"-1/2" below it. Now plug it in and very slowly start opening the screw. This will remove about half of the trapped water in the airline and the rest will evaporate over about a week.
The sure way is to open the skimmer, pull the pump and drain any water in the airline, follow the same details regarding water height when reconnecting the skimmer.