The mk5 gti engine is a bit renowned for ticking/tapping/diesel noises. The injectors are very tickey and this is totally normal but can mask other sounds.
The main concern is a "diesel" type noise at warm idle, often not present when cold. This can be the small timing chain & tensioner on the right hand end of the camshafts (looking from the front of the car) wearing. I had this and solved it a couple of years ago.
More recently I've been chasing a bit of a tappy rattle at warm idle again and had the car looked at by an indy, RTech and AKS in the last 6 months. The first 2 couldn't pinpoint but said it didn't sound like anything to worry about (its very quiet but hard to un-hear once youve caught it). AKS also said nothing to worry about but it could be cambelt/water pump related, so I'm having these done as they are due in less than a year anyway (cam belt was done 3 and a bit years ago, but don't think waterpump was done at the same time).
Other suspects could be oil pump/balance shafts at the bottom of the engine, evap purge valve near the charcoal canister or the fuel pump cam follower, again top right near the timing chain cover. I think a faulty N75 boost pressure control valve can make a racket too.
Impossible to diagnose by text. Try probing around the engine bay with a stethoscope to see if you can narrow the area of origin down, or get to someone who really knows these engines (i.e. an RTech or AKS, not just a general garage) to take a look!
