hot water boiler

Hello all,
I have a Triangle Tube Solo 110 boiler with 3 zones. (DHW, 1st and 2nd floors). Recently, I've begun to notice a tremendous amount of noise (water slooshing and swishing) in the 2nd floor pipes when that zone calls for heat, as though there is a lack of water in the system. However, the boiler reads a very ample 20psi. There is a Spirovent valve in the system next to the boiler which I assume should alleviate an air bubble problem but this loud and obnoxious water slewcing sound on the 2nd floor persists. The system is in its 5th season and this is the first time this has been an issue.
Has anyone else encountered a similar problem.
Thank you for your thoughts on this.