Engine knock on idle only

[Wees]

Hi guys, I have a 2004 Impreza wrx wagon owned it for a few months now first Subaru I’ve owned, but the past few weeks I’ve noticed a slight knocking noise from the left side of the engine on idle. Obviously first thought is to panic but after reading some other forums a few people have had the same issue. It wasn’t doing it when I bought it in the back end of the summer, it is only on idle and goes after it’s driven or reved but comes back when turned off and restarted. Really baffling me so thought I would ask for some help, anyone got any pointers? Many thanks 

[Tidgy]

Get a specialist to check it, could be nothing serious, such as a belt tensioner, or could be bearings (they tend to be noisey at lower revs before they totaly fail)