Air in pipe I Think

Problem ..... Pipes making awful noise at first turn on of hot taps
Knocking pipes when the kitchen hot water is turned on
But turn the bath or sink hot on which is on the same floor and next to the kitchen and the knocking stops
both the bath and the sink in the bathroom have mixataps and the pipe to them reduces down to about an 8 m/m connection
Question 1 are the reducer bits the problem?
Qustion 2 any idea what the remedy is? .....
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file
Upload is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.