How can I fix my Vaillant Ecotec Plus 428 boiler problem?

When the outside temperature is very cold, and the boiler fires up to heat domestic water and/or the central heating, after about 15 minutes, a vibration/rattling noise is generated in the boiler, which can last for 10 minutes or more. It is an intermittent fault and can't easily be reproduced and only happens in very cold weather.

My maintenance company have been unable to resolve the problem, although I've played them a recording of the noise.
On their last visit, their engineer reduced the pump speed and adjusted the bypass valve ( I have an S plan setup). This appeared to clear the vibration problem but caused the status code S 35 problem, which prevented my radiators heating up fully. With the pump speed returned to its original value, the S 35 problem disappeared but the vibration returned.

Has anyone else experienced this problem?
Any comments/suggestions as to the cause and cure would be greatly appreciated.