Unusual noise on 2004 Subaru Impreza WRX STI

I just recently bought a 2004 suburu impreza wtx sti with around 80k miles on it. Carfax was good, had a mechanic inspect the car upon sale. Just yesterday it started making a really loud fast clicking noise under the hood. I mean, its not just a subtle noise, its almost too loud to drive now, cop friend of mine said i would probably get pulled over with its noise. Any clue as to what this might be? As soon as i turn it on, the loud clicking/clanking begins. Doesnt matter which gear im in, it gets louder as I start driving which is hard to believe seeing as its almost unbearable in neutral. Manual transssmission btw haha. Thanks guys

Asked by for the 2004 Subaru Impreza WRX STI
it could also be a rod knocking if it's that loud! but this is very unusual for these engines but i would think it's more internal as those knocks are often louder and def NOT an exhaust leak i mean come on now who would think that?!
1 more answer
Its hard to guess at a noise without being able to hear it first hand. Check the engine oil level off course. Pretty good engine but a lot of these are driven really hard. Could it be an exhaust leak from one of the manifolds? Get it checked as soon as possible in case it is really serious and will do more damage.