Air trapped in heat exchange - conventional boiler

Hi, firstly thank you for any help you offer.

Back story - We recently drained our heating system, we have a conventional system with a Potterton suprima boiler. While the system was empty I very stupidly turned the hot water. The system chugged an I immediately turned it off. Upon refilling the system we filled it with sludge treatment and the radiators heated up well an the hot water runs perfectly BUT whenever I turn the hot water on to heat up the boiler boils like a kettle, it sounds full of air. We have bled the radiators and main bleed by the boiler over an over with no air released. Radiators are hot from top to bottom.

Question - Can air remain within the boiler (heat exchange) even after refilling the system. Would draining the system again fix the issue. Don't want to empty it to find the issue is specifically in the physical boilerm

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