I'm just wondering if the macbook I have 2.4ghz has a dodgy soundcard or if it is a cheap soundcard with crappy artifacts. When I wear headphones I can hear pops, clicks, water trickley etc.

Is this normal?

I get it too... Have googled it a few times, and seems it might be an issue, but Apple are yet to acknowledge it.
Mine is generally when there is no music playing, and from the googling, it seems it could be the sound card going to sleep when not in use and then starting back up again. I've learnt to live with it.

EDIT: I have a MacBook 2.2GHz, and the popping only happens (or is only noticeable) when I have it connected to external speakers or earphones.

Happens with my macbook pro 2.33ghz too.

I have also read it happens when the sound card turns on and off. You don't hear it when playing music etc because the sound card is staying on. Usually only happens when speakers and headphones are plugged in and not out the actual speakers.