Noisy automatic bypass valve or pump?

[Debs]

Hi I have a vaillant ecotec plus condensing boiler which is in the kitchen with an indirect unvented centerstore cylinder in the airing cupboard. All has been well for 18 months (the cylinder was installed in April 2018.) The last few months there has been noise in the airing cupboard which sounds like a sort of whistling noise when the boiler shuts down and the pump speed starts to reduce (I believe this is the pump overrun). 4 engineers have tried to fix this problem by the first one turning down the pump speed (this took too long to heat the house and as we have microbore fittings restricted the water even further), the second increased the pump speed to it's original position but turned the Automatic Bypass Valve down, the third said it needed new AB valve which was duly changed. Still no joy so called the engineer again and he said that the ABV had been left fully open and so he turned it down. However shortly after he left and as the heating was going off the noise is still there. He said when he left he may have to change to a manual bypass. I am confused as to why this was ok for 18 months but now we are having problems. Any help would be appreciated as it looks like I have to recall the engineer again for a 5th time. Just to add the engineers have all tried to fix the problem.

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Engineer coming Wednesday. Hopefully 5th time lucky.