Baxi Solo whining & groaning?

..like a pair of alley cats in the night.. very annoying early in the morning when it comes on!
Service man says that there's probably an air leak around the fan assembly (previous guy pulled the plate too hard and buckled the top seal). Tried to re-fix the top seal with selftappers (original security-type fixings came out) but still whines & whistles. Can this be sealed with mastic and if so, what type?