I think your fan unit is caked with dust which makes it unbalanced (the source of the low thunk noise). You’ll need to open the system and try cleaning the dust out. Use s small soft paint brush and a can of can’ed air to blow the debris out. If that fails you’ll need to replace it: [https://www.usedmac.com/products/922-9358|27” LED Cinema Display, Apple P/N ''''''922-9358][br]
To get into the system you’ll need to take the display off. The display is mounted in the same way as the [guide|7551]