2002 1.8 V40 manual - sloshing noise from engine?

[wst]

I've noticed that after the engine has got up to temp and been running normally on a drive (usually motorway with a town at each end - commute), when slow or stationary the engine makes a sloshing kind of noise. My dad (who drives the car) doesn't seem to care but frankly it sounds horrible so I'd like to hear some ideas what it could be. Fluid levels are ok. It's been for a service since the noise, but they obviously didn't remedy it through their actions. It's on around 110k miles so far.