Return making Bubbling sounds

[Gools]

One of my returns is making a bubbling sound that's driving me crazy. I have a 4 way, and it is the furthest one from the 4 way that makes this noise. The other 3 don't make this noise. Any ideas on how to make it stop.
Thanks

 

[Gabriel_T]

I'm not sure but I think that means air is getting into it. I would check all the fittings and the pump for cracks or loose fittings.