Alpha combi whine

When I turn off the hot tap, after running hot water, whilst the central heating is ON, I get a loud high pitched whine coming from the boiler. This can be stopped by turning the cold tap on and off. I suspect that the divertor valve may not be returning to its correct position once the hot water demand has ceased. Can anyone confirm, or indeed discredit this theory?

this will help, 3, 4 & 6

The boiler is an Alpha CB28 combi. The system pressure is ok. It is approximately three years old and has been making this noise for quite some time . There is no problem getting hot water when the heating is off. When the heating is on and I turn on the hot tap I get lovely hot water, but when the tap is turned off I get this high pitched whine/squeal noise. To make it stop I have to briefly turn the cold tap on and then off. It also happens when the washing machine is on and is a pain in the buttocks.

I also have one radiator which I can hear the water running through, it is quite noisy compared to the others. I added said radiator myself and it has always been considerably noisier. Like the other rads in my system it has a danfoss thermostatic valve, which I assume is bi-directional (arrows pointing both ways).

I have drained and refilled the system a couple of times but have always added plenty inhibitor fluid.

One to call Alpha on I think. Would be interested to know what it is.

Turn the lockshield (valve at the other end) down on the noisy rad. Try it open just a tiny bit.

This problem may well originate elsewhere in the system, and the most likely source is a section of air filled pipe. I have had, and solved this problem on several occasions.
If the stoptap is not fully open, the problem is likely to be solved by opening it. if this does not cure it, check the stop tap for a loose jumper or worn washer- you may need to turn the water off outside to do this. When you turn a hot tap on, any air trapped in redundant, or upward looped, pipe expands, When you turn it off, especially suddenly, a very small amount of water continues to flow to repressurise that bubble. Any sudden change in pressure causes items such as tap jumpers, or loose pipes, to vibrate which propagates shock waves through the pipework, often causing noise at places distant from the source.

Thanks for the responses. Will let you know how I get on.