Need a lot more info, is it a metallic noise? A thudding noise? Squealing? Does it sound like it's in the actual fender well itself, or the engine bay on that side? Closer to the cabin, or the nose of the car? Is it intermittent when it happens or constant? Is it French? Does it resemble the screeching roar of Godzilla in anyway?
From the info I have, I assume you ran over a small homeless child and kicked him up into your fenderwell, and when you put the motor under a lot of load the turbos get hotter and it pisses him off, so he's knocking to tell you it's getting too hot under there. Toss a few bottles of water and some chicken nuggets under there and you should be fine.
