Driving a Clio into the ground in the true spirit of the phrase LOL Nice....I know you're a Clio lover or something but IMHO that's how all circa 95 Clios should end their life. We had one and it was the 2nd worst car we ever owned, behind a Rover 45.
Double check your sunroof, try pushing it upwards at the back, naturally if it lifts up then it isn't secured by the lifting mechanism, only sitting in it
It is a possibility:
There's a red tab that locks and unlocks the sunroof on my Espace, if your car has it I'd check it to make sure it's OK.
I can see how the sunroof could make noise when open as the rear has a tiny bit of free play on the system I am talking about, may be possible to get something soft and durable in there to take up the free play without affecting the winding operation.
The water glugging noise could just be the heater system, the erm.......heater matrix. Just a thought, dunno if it would be any more noticeable in rain.
Hope your new Clio proves to be a decent one, I heard the newer Clios are a bit nicer.