Ive heard a noise very similar to that before and against my advise the chap carried on driving it and a day later his water pump with the viscous fan on it let go on the motorway and took out his radiator, radiator shroud a couple of coolant hoses as well as the tensioner and it also dented the hell out of the bonnet from under side.

So don't be tempted to delay finding the cause, I'd whip the belt off and give the water pump pulley and good wiggle and spin