Sloshing sound behind dashboard - General cooling system advice needed - Superb mk3 2018 3v5

Recently I've been getting a sloshing sound coming from behind the dashboard when I turn the ignition off - and sometimes when I turn the ignition on but don't start the car. When switching the car off, if I wait a few moments, I get a sloshing/draining nose which eventually fades.

The car seems to get up to temp okay and no obvious signs of overheating. Paranoia tells me that I can smell burning coming from the engine but I think the oil smells okay.

I have tried bleeding the system - getting it up to temp and then whacking the heating on and ensuring warm air is coming out of the vents.

I noticed that coolant is being returned to the resevoir but only upon revving the engine. Is that what's supposed to happen? So basically, when the engine is up to temp and idling, no coolant is being returned and that strikes me as odd. Could the revving be forcing collant through a potential air lock?

Does anyone have any ideas about what this could be?

Thank you in advance