might be the bottom cover of the fan.
mine having some sort of sound like that..
have to open the bottom cover, and retighten.

the center middle cover, just need to twist it to open.
the bigger cover got 3 screws under the center middle cover
btw, this is my fan la.. but seeing ur model looks like the same design bottom part. 

in other matter, last time i bought 2 box (4 unit fans).. and bought another single 1.
but the speed for the 1st 4 unit kinda slow than the single 1 unit i bought.

opening up the circuit, reveals that the earlier 4 unit using 450V 2.0uF starting cap.. but the latest one 500v 2.0uF.

[attachmentid=10371980] [attachmentid=10371981]
i have replace/upgraded the capacitor from 2.0uF to 2.2uF for the 4 unit 450v.

[attachmentid=10371982]

now the fan much more faster 

slow on All Speed or certian speed setting?

noise, u have to check, what type of noise.. grinding noise, nyet2 noise..ringging noise..

Uusally the 0.68uF capacitor also need replaced. The default value is 1uF if you check clearly in PCB, it print 1uF, 450V