I'm getting this sloshing too, on my V '99 1.6 Sport Alize Megane, but she's also over-heating, and going through coolant at a heck of a rate.
Are the two linked?
I had error light come on on Saturday, and the temperature needle was nearly at max - I cranked up the heater, as I couldn't pull over, and was only a mile from my destination. But little or no heat came through - it turned out she had boiled dry. My dad filled her up, after she cooled down, but it bubbled, and went down, and bubbled, and went down - she took over 6 pints.
She's running ok, but she's still drinking lots.
I can't afford to take her to a garage until the end of the month, but I need her for a trip tomorrow.
There's no sign of steam coming from her exhaust, nor from the cap on the water reservoir. I don't know where the head gasket is, so I wouldn't know where/how to check that.
I've checked her
, via the dipstick, but not via the filler cap - will check that tomorrow morning.
Any other pointers?