Whistling tado

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Now and then, mostly early mornings and late evenings, after the temperature is set to change, one of my Tado whistles loud and the other softly. Another three do not whistle at all. If I press the Tado round front piece inwards, the whistling pauses, then resumes when I let go.

I have a video of this but I am not sure how to post it.

I am pretty sure that this is a different issue from another, already discussed in the platform, which apparently had something to do with the exit valves. That is not the case here.

Already let all air out from the system and have made sure all the fixings are tight.

I am running out of ideas. Can you help please?