1994 Honda Accord LX

I don't really know how else to describe this noise other than it's a pulsing low hum. It sounds a lot like a wheel bearing problem I had recently on my girlfriends 2004 Pontiac GP. The only thing that makes me think it's not even close to a wheel bearing problem is that the noise goes away when I let off the gas and it's mainly only apparent when going 50+ MPH. What could cause this?