Point to note: I purchased some cheap fans that fit. They worked fine for a couple of months and then started making some noise but then being quiet. After checking the rear of the unit the reason they were not making noise is because they were not spinning.
Replaced with the original fan and the noise persists for 30s and then is quiet (but still spinning).
So, I recommend high quality replacements rather than cheap. Sunon has always been a good choice, as has been Papst.