My computer liquid cooling system making a rattling crackling noise how do i fix it?

ok basically my computer has recently started making an annoying rattle crackling sound almost sounds like sand rattling in the tubes. i've looked everywhere online on forums tech support and everything says its probably a lose screw. i've gone through taken the entire thing apart and put it back together and cannot fix this rattle it gets worse if i play a high gfx game. and its minor but irritating when i'm just typing or browsing internet. i can't seem to find out if its the fan or if its the liquid cooling part. the computer runs with no problems other than this sound is so irritating. this computer was silent when i got it then it just recently started. i really can't seem to figure out how to fix it. is the liquid in the cooling system low or something is that even possible? i'm not sure but its like a loud pive fizz from an open soda next to you and nothing i mean no amount of background noise can drown it out. please offer any help if you can i really don't have the cash to replace a liquid cooling system.