Nope, no way to fix it without voiding the warranty. Hope you kept the box they sent the replacement back in, because odds are real soon you'll be needing it. A loud fan is usually a fan that's going to die imminently.
It could die tomorrow, it could die 6 months from now, no real way to tell. But it will likely die sooner rather than later.
I sent my Xbox 360 in, and they gave me a replacement Xbox. and this Xbox is noisy, I think its the fan inside the xbox.
I dont have it running for long or anything. Right when I turn it on its loud fan comes on. Its about three times louder than a normal Xbox 360 fan.
Is there a way to fix this? Or do I have to live with it?