Noise during modulation

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Hi, we have the baxi 830 combi using opentherm with tado and Drayton TRV's.
If radiators are balanced they all heat up properly and radiators are quiet until the boiler modulates down as it starts to approach the target temperature.

As soon as the boiler modulates down the longe rad for eg starts making a motor running noise. The only way of stopping it is to open the lockshield fully (which then will mess up the balancing)

Is the only resolution to leave the lockshields fully open? Where does the problem lie?

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