why does my boiler go squooooooooosh?

i have a valiant condensing system boiler on a sealed system.

why does it go sqqquuuuuuuuuoooooooooooooooossssssssssssssshhhhhh.
at random.

i've recently had a new heating circuit put in downstairs.
the plumber used a fernox flushing liquid and has treated the system with inhibitor.

the boiler made th enoise before i had the work done.


so any ideas????


thanks

sa?a