2 Memphis prd 1000.1 amp fan
 

I have 2 Memphis Prd 1000.1 amps they look the same except for the amp cover plates. i've had them both hooked up for some time now and the other day i noticed that one has a loud noise coming from the fan where the other one doesn't i thought this is a cause for concern so unhooked the one and just running on the quieter one now.

just wondering if this is something i should be worried about and if it is should i be taking it in for service or is it as simple as the fan needs to be oiled

thanks for any help