Skimmer Loud knocking noise, was overflowing put it on stand does fine at first but within hour starts making this loud knocking noise

[ethol]

loud knocking noise, skimmer was overflowing water so put it on a stand does good but within an hour starts loud noise, goes away if i put my finger and block water going in the pump

 

[FUNGI]

check to make sure impeller is sitting correctly.......

 

[ethol]

thank you! i checked the impeller and noticed it had too much give up and down so i changed it with a sicce impeller from an extra syncra and is working like a charm!