HP laserJet 5200dtn making buzzing/humming noise after printing

Hi,
We have about three of these units in our office and one of them has recently began to make a loud buzzing/humming noise during and after printing a document. THe noise last about 5-10 seconds after the print job completes.  I can also make the printer buzz by removing the duplexer and puting it back in. The printer seems to load and then the buzz noise happens, again for about 5 seconds. I am aware of normal noise from these printers and this buzz is not normal.
This noise gets louder as the day goes on. Could this be a fan motor going bad?  We need to do somthing to get it quieter.
Any help or suggestions?
Thanks
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any suggestions?