Technical Strange noise from throttle body

[Mark Saunders]

Does anyone know what this is? I noticed when driving my wife's 2000 100SX this evening that it has a very slight misfire just above idle. It's not serious and there are no warning lights. I thought I'd take a look under the bonnet and noticed that if I rev the engine slightly, as it settles back down there is a kind of slurping noise coming from the throttle body that seems to time exactly with the misfire.

I don't see any sign of an air leak and it doesn't really sound like one. There are no other symptoms.

Any ideas?