I just noticed it today. It takes 2-3 minutes of driving for the noise to kick in, and when it does the noise is very aparent from the cabin. It's definately coming from the engine and when the throttle is pressed, the noise seems to worsen. If I'm just idling it runs fine, but as soon as I start to drive it'll act up again. The noise also goes away if I clutch while driving. Just before I turned the car off I tested it in neutral, and the noise seems to be the worst right around 2800 rpms. The car has 155,000 kms on it, that's about 97,000 miles. One more thing to add is the car sees to drive fine regardless of the noise.

 

As you can reproduce the noise in neutral, try to pinpoint the source. An inexpensive engine stethoscope would help.

In neutral, can you also make the noise stop by pressing the clutch? Have you checked all of the drive belts, drive pulleys, and idler pulley?
Other possibilities are a cracked exhaust heat shield and a bad input shaft bearing in the transmission.

 

The noise is there regardless if the clutch is pressed in or not when it is in neutral. I don't think it's an input shaft bearing, but I'll check the heat shield and my belts tomorrow. Anymore suggestions would be helpful. Thanks

 

If the noise is from the belt area, take all the belts off and spin the parts by hand. Not finding anything obvious, start the engine with the belts off. If you still have noise it is probably the water pump or timing belt tensioner. Make repairs immediately since if they seize up the timing belt will strip and you risk major engine damage when that happens.