I have a La Marzocco Lina.   Making noises when pulling shots and even more when back flushing.    Hard to figure out exactly where the noise is coming from, but sounds like cavitation of some type. A buzzing or growling, makes me think “old pipe sound”.   Not sure if that makes sense.   (tai game dua xe)
The weird thing is that, when I had the top off, and back flusing, (back flushing makes the most noise) the pressure relief on the steam boiler would spirt.  The boilers, I thought were independent?  Brew boiler affected by the steam boiler?   I think where they connect is at the water inlet…….   What could be wrong here?  It is about 15 years old, and been well maintained.  I am thinking mineral build up or?