You will need to elaborate on what you've heard since
strange doesn't help narrow anything down. Did you hear sloshing of liquid inside the AIO's tube/pump area? Also, if the pump wasn't working, didn't you get any thermal issues or failure messages while on your system, if it was powered up(and running) for the past 1.5 years?
Make and model of your AIO and it's orientation? Might want to include a picture of the layout of your system. Host your image(s) on Imgur and then parse a link here for us to see.
This sums up what's going on in my head right now...