gurgling/bubbling sound from AIO cooler

my PC was a prebuilt which I've had for over a year now, and from the first day I got it the AIO cooler has gurgled quite loudly upon start up, and then occasionally every 10-15 minutes when I'm using it. I monitor my temps frequently and all seems fine there, so is this something I should be concerned about?

I have also contacted the company I got the PC from about this and they've reassured me it's 'normal' and nothing to worry about..