What the pilot said was true.

Ideally, the prop heat is turned on before any ice accumulates and the ice is slung off before it gets to the point where it makes any real noise in the cabin.

However, when the props are picking up ice at a good rate, the exact situation you described will occur. The props will become imbalanced, which causes increased vibration, and ice will bang against the fuselage as it becomes dislodged.

The solution on the Q400 is to speed up the propeller speed to 900 RPM (850 is the normal cruise setting) to increase the centrifugal force and sling off the ice more effectively. Hopefully, this forces the ice off the props while its a bit thinner sparing the passengers in rows 1-3 from having to experience those unnerving banging sounds every so often.

Checko