When driving our 2001 montana, the noise gets really loud. Like a loud humming. What could be causing this noise? We had one wheel bearing replaced on the front left wheel which was a little loose, but now the noise continues.

"Loud humming" could describe many of the noises that your minivan may make. Let's start with the basics: do you hear the noise only when the van is in motion or also when the van is stopped? If you can hear the noise even when the van isn't moving, you're most likely looking at something in the engine compartment. Your serpentine belt and the alternator bearings (and, to a lesser likelihood, your power steering pump) are all sources of noise "in there." An A/C compressor on its last legs may also vibrate and cause a humming sound, though that's usually described as more of a "growl."

If you can only hear the noise when the car is in motion, the next question is whether it changes pitch/volume due to engine speed or due to road speed? For example, does the noise get quieter (momentarily, perhaps) when the transmission shifts gears? Or, does the noise simply get louder/change characteristics in a linear way as the van gets faster and/or slower?

Depending on the mileage on your van, the transmissions have been known to get quite noisy after a while, especially if you are doing sustained high-speed driving. The solution for that particular problem isn't cheap, as it involves replacing or rebuilding the transmission (the shift bands may be fine; the noise is caused by bearing wear, seal failure, and other sundry issues that still require pulling the transmission). A less expensive problem is that you might have a problem with one of your tires--a tire with separating belts may make a noise like this as it slowly tears itself apart. If you hit something hard with the undercarriage of the van, you might have a drive shaft that's slightly bent and hence is causing some sympathetic vibrations in the car because it's out of balance.

Depending on the cause, your humming sound may be one of those things that comes with driving an older car--or it may be a sign of the imminent failure of something serious. It's worth getting things checked out again to try to find the cause of the problem. You'll feel a lot better knowing what it is, even if it's something you choose not to fix.