Thumper gurgling

[MementoMori]

Hey friends hope all is well. I have a 20gal stainless steel tank as my boiler and a small beer keg 7 gallons for my thumper. I'm using electricity for the heat source and just finished my sac run. My thumper has sight glasses and at the end of the run I noticed something new. The pressure in the boiler was sucking distillate then pushing it back into the thumper. This was concerning obviously. My boiler is thick stainless steel as is my thumper. I'm not concerned about implosion in that regard. My question is can I add a pressure relief to my boiler? I have a 1.5 tri clover fill port that I can use for the valve. Which valve is recommended that I can bleed off the pressure?