I've got an early 15"MBP, arrived Feb28, that has suffered a dead left fan and two batteries. The first replacement battery was before Apple knew what the problem and affected lots were and was from the bad bunch. New battery works long time. I tried smcFanControl, but prefer Fan Control Pref Pane - more adjustable.
My MBP is virtually silent until the fans get up over 2500rpm, but no hum... When the left fan died, something had shifted, the fan sounded like baseball cards in bicycle spokes. All repairs/replacements have been warranteed and I'm happy with the MBP now. Fasssst and cooool.
Does your "hum" change with fan speed or is it constant? If it's constant it could be a 60cycle hum from your mains power(unless you're outside the USA, then it might be a 50cycle hum...) although I've never heard of an Apple laptop having that problem, usually caused by a bad ground in the audio circuitry.