2018 Citroen C3 screeching noise when driving and sound after engine switch off when air condition is on

The air con takes a big electric load when used . If your battery is low , possibly from short journeys / not used a lot , when switched on the alternator works harder and possibly you're hearing belt slipping . Check your coolant level when cold , bubbling may be caused just before engine coolant fan kicks in but keep an eye on it as other causes also .