Replace balancing valve/setter with auto bypass valve to reduce noise?

Problem
Each morning, when my boiler switches on to heat up the hot water cylinder, we hear some gurgling and minor pipe rattling sounds, the sound is a minor annoyance, it doesn't last longer than around 10 seconds.

On inspection of the system, I found there is a TacoSetter inline 100 balancing valve on the bypass, which is blackened and impossible to read the flow rate, but I can confirm it is not blocked and so the bypass is functioning.

I believe the air is getting trapped in the cylinder coil, because there is insufficient flow to 'purge' it back around the system and escape. The reason I believe this, is because if I close the Tacosetter and set the pump to speed 3 (normally on 2), with HW demand on, I can hear the air purging out of the cylinder and the problem goes away for a week or two.

I was reading the boiler manual, and it says an automatic bypass valve is required, but there is not one fitted anywhere. I believe if I removed the TacoSetter and replaced it with an ABV, then the issue will no longer occur, am I correct?

Background

• Open vented system
• British Gas/Glowworm 330 Boiler (Downstairs)
• TacoSetter inline 100 (3-12lpm)

Annotated version:

The bypass is fitted to ensure a minimum flow through the boiler when the valves are closed and the pump is on overrun to remove residual heat from the boiler.
Check around the De aerator connections with a magnet.

Terrywookfit said:

The bypass is fitted to ensure a minimum flow through the boiler when the valves are closed and the pump is on overrun to remove residual heat from the boiler.

Click to expand...

Yes, I understand that.

Terrywookfit said:

Check around the De aerator connections with a magnet.

Click to expand...

Why check the de-aerator with a magnet? What will that achieve?

Blockage in the cold feed and vent,
There should be no trapped air.

Terrywookfit said:

Blockage in the cold feed and vent,
There should be no trapped air.

Click to expand...

Terry, I really appreciate the reply but I am failing to comprehend! Can you elaborate on why a magnet will help to alleviate a potential blockage in the cold feed &/or vent?

I am perplexed as the CH system shares this pipework and suffers no issues (i.e. annoying sounds!). Thanks

You use a magnet to check for magnetite. https://www.idealheatsolutions.co.uk/do-you-have-a-sludge-problem/

The sludge which causes the blockage is slightly magnetic. If a magnet is attracted to the air separator (that's the small cylinder in your first pic), which is made of copper and therefore non magnetic, it's an indication of a potential blockage. It's also sensible to checkthe pipes attached to the separator.

A pic showing the complete HW cylinder as well as everything else would be helpful. .

The magnetite (Dissolved steel ) can solidify and partially block the pipes usually the cold feed where it joins the flow.
When the pump first starts it causes air to be sucked down the vent.
If a magnet is attracted to the area it is a sign of blockage /restricted flow.