Baxi Solo HE resonates when it modulates down?

Hi, I have a couple of Baxi Solo HE a boilers in some of my properties. After about six months from installation, they have both begun to resonate when they modulate down. If I put my finger slightly over the venturi air intake, it stabilizes. I swapped the fan unit with one from a brand new boiler and it stopped for a couple of days & then started again. Any ideas? Gas Valve? Flue? HELP!