How to fix loud foghorn noise from shower when going from hot to cold water?
With the flow on the cold being the issue it sounds like water hammer. I have seen extreme water hammer with very tiny flows but it is much more common with high flows.
The 2 solutions I know of for water hammer are.
Strapping the pipes that are chattering, if you have access a couple of straps to keep the pipes from chattering will quiet it down.
Add a water hammer arrester these are devices you install on the water line that have a chamber spring loaded or air chamber that cushion the flow issue. For best results they need to be on the line that is chattering but I have seen one installed at the toilet fix a shower &amp; sink issue. These devices cost between 10-20$ in the US and since most plumbing is not accessible would be my first thing to try.