Hi,
I have had the same problem. The fan on my TL550 has been making a terrible racket. I have been searching the web for advice on how to change / access the fans in the hood. Even contacting BOYU was a pointless excersise. Their customer service is a useless as the instructions that come with the tank.
Anyhow, i gave it a go at the weekend. Took off the splash cover, and then undid the screws in the silver reflecter below the lights. In order to fully access the fans the whole reflector underneith the lights would also have to be removed. I just wanted to take a peak so undid only the screws at the other end. I was able to lift the reflector gently and see the fans. I then sprayed compressed air into the fans to try and clear any dust that had built up. After putting it back together my fan has been silent. But if it becomes noisy again i shall take the whole thing off and replace with new ones.