As you probably noticed, the fan on the wireless model of Juniper SSG5 is known to be noisy over time and does not last very long (maybe a year or two if not in a dusty environment).
Even though the device seem to be running fine fan less, I would not risk it on a long period of time… if you push the wireless radios hard enough, it may over heat.
You can easily change the fan by removing the 4 screws located on the sides at the back and front of the device. Before sliding off the cover, make sure that you unscrewed the wireless antennas and the RP-SMA jacks. Once the top cover is slided off to the back, lift it very easy and make sure the RF cables/connectors are removed. You can then disconnect the fan cable and unscrew it.
The genuine fan model and specs are the following :
SUNON GM0503PHV2-8 30x30x15mm 3015 5V 0.4W Mini DC
This fan can be found easily on eBay and probably your favourite electronic shop with this model number (GM0503PHV2-8).
DISCLAIMER : This procedure/information does not come from Juniper and would probably not be encouraged/supported by JTAC.