Ok well I'm at a loss at this point so I'll fall on the mercy of NASIOC.

I have an 03 WRX w/ just under 45k miles on the clock

Engines mods:
Deadbolt Monster TD04
Ebay style TMIC
STi Pinks
Perrin fuel rail
StoneMountain Racing headers and uppipe
3" downpipe

Not really sure where to start...
So here's the issue. Just as of about 3 days ago the rex started making a fairly strange sound coming from the engine under load. It sounds kind of like a jackhammer i guess, sort of like a pop every other revolution, it increases in frequency as the engine speeds up and increases in intensity with large throttle opening. It kind of sounds like a VTwin with a quiet exhaust. From the frequency it sounds like it's coming from just one of the cylinders... or in sync with one of the cylinders I guess depending on what's causing it. The sound only occurs with the thrrotle at about 5% or greater opening and it seems regardless of boost. With the engine or gear or not, it doesn't seem to make much difference.

It doesn't seem like it's down a huge amount on power... it might be spooling a little slower, and it's hitting about a pound below normal peak boost, although I haven't tested it a ton and it's not usually super consistent regardless.

The only incident that might relate to it is about 2 months ago i hit the headers on a rock driving along and it put a little dent in one of the primary tubes... it didn't puncture it or anything though and the sound didn't start occuring until quite a bit after but possibly related i suppose. I tried tightening the headers down today but it didn't make a difference.

It doesn't seem to be exhaust related, i had a friend of mine plug up the exhaust at idle and the sound stayed exactly the same.

I'm not really even sure what kind of info is pertinent here so go ahead and ask any questions you think might help shed light :X I'm in the dark here so i'm eager to atleast get ideas on what to look at.

Someone suggested it might be a stuck intake valve but I haven't heard of that much on these cars... i ran some valve cleaner regardless but it didn't make a difference (although that doesn't necessarily mean that wasn't the problem).

Oil pressure, oil temp, coolant temp, idle vacuum all are normal. My EGT sensor went haywire a long time ago so it hasn't been in for a while :X Again, power might be a little down but it's not an enormous amount, it could easily be psychological.