2004 Dodge Sprinter loud humming noise

2004 Dodge Sprinter 41000k miles

I'm about to buy a used Dodge Sprinter that has been converted to an RV by Pleasure-way. I test drove the vehicle and was surprised to hear a rather loud humming noise that began after a few minutes of driving. The noise continued for 1-2 seconds after the engine was shut off. When the key was in the auxiliary mode the noise was continuous. The owner thought that it was perhaps a fan but essentially he said that the noise was always there and he was used to it. Can anyone tell me if there is a problem.
Thanks
scott. Follows@acadiau. Ca