Chattering from front right of engine, Defender 300tdi.

Went back and had a look at this today. Lying underneath with it running there's no wobbling or anything whatsoever from the crank pulley that shouldn't be there, same with the fan idler bearing and pulley, that's solid and with the engine off I've tried to see if I can get any movement from either of them but there's none. The sound is no longer noticeable from inside the cab and you can only hear with the bonnet up and standing listening for it. It sounds like a trains wheels on a track, it's quite rhythmic but doesn't seem to match up with the belt or pulley speeds. Im 100% it's still coming from the water pump, infact I'd put money on it, but I don't know what it can possibly be