Can somebody figure out what's wrong with my filter?

It's the waterfall style filter from BOYU (this is the brand available in my country, so if you recommend me any other brand, sorry I wouldn't be able to get it). I'm using it for my 2 gallon betta tank. Last night it filtered my tank well. There's no strange sounds and my tank's water got clear. But today, I realized that the filter is making strange sounds like "pep pep prep pep prep pep pep" as if it's having a hard time sucking the water from my tank when the water is circulating well. Anybody knows what's wrong?