rhythmic hum from engine at highway speed

[rtom]

Does anybody else hear this?

My manual transmission 3-door LB with about 11000 miles on it is making a humming sound that modulates with a cycle time of 3-5 seconds. I don't know the right musical terms, but here's an approximation:

HUMMMMMMmmmmmm HUMMMMMMmmmmmm HUMMMMMmmmmm

where the all-caps means louder and small-caps means less loud. It's a vibration of some kind that is coming from the front of the car, probably the engine or transmission, and it only occurs under load and at 55+ mph. If I push in the clutch, it stops.

I know funny little noises can be very hard to diagnose, but thought I would ask. I don't think I heard this when it was new.

Also, after the second oil change it is less loud.