Is it a mains powered fan or DC? I had a 5328A that was loud as hell with a mains powered fan. Decided to shift it after a bit because it wasn’t worth modding (it was sick anyway).
As capt bullshot wrote: It's a DC-Fan, Papst 12V, 2.5W, Type 612; A 60mmx60mmx25mm ball bearing type with about 3000 to 4000 RPMs I guess. I've replaced one of those in my HP 6643A with a Sunon Maglev DC 12V 1.5W fan (MB60251V1-00U-A99). CFM was a wild guess but it seems I guessed quiet good. Nevertheless I also ordered another Model with higher CFM to test with it as the exhausted air gets quit warm (as with the papst).
The annoying sound of the papst is due to the ball bearings. The Sunon is much quieter -only the sound of the air flowing is there and that can't be avoided in this application.
And yes, the transformer is humming relatively loud -you have to like the 50Hz sound -or keep the papst.
But all things considered I like this beast and wouldn't give it away. Fitting front binding posts is in the queue*.
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* - meanwhile the paint job is done and the plate fits the colour of the 6632B quiet well.