Same thing happened to mine, but I'm sure it "came loose" progressively after a service, cos I didn't notice it for some time. Once it got bad enough that I noticed the noise, I too had all kinds of thoughts about what it might be... The water pump was one of the most obvious.
Anyway, cutting the long story short I finally found the loose plug (middle of the front bank), got it tightened up, and the engine has been fine since then. (Touching wood...)
It surprised me how little impact on the driveability it had too - I had barely noticed the drop in performance and the tickover was not noticeably worse either.
Matt.