A few people have brought up good ways to replicate a dolly. Just by leaving a closed or folded-up tripod attached to the camera you get a lot of nice weight off the bottom that lowers its center of gravity and can make even your handheld shots smoother.
I've taken shots out the back of [slowly] moving vehicles to truck alongside a walking actor, and used one of those rolling carts for a semi-circular surround shot (along with some WD-40 to grease up the wheels
. Using a wheelchair sounds like a great idea; my folks have one at their house from when my grandfather lived with them, but I've never thought about using it before! Maybe I'll go make an "unannounced family visit" run this weekend. heh heh...
Think about other things that have wheels, both mentioned above and others - rollerblades, skateboards, wagons, bicycles, etc. You'll be hard-pressed to find many that track completely silently, but those hybrid/electric cars are getting quieter and quieter these days. Of course I'm not suggesting that you film a high-speed chase hanging out your car window. And definitely not if you're also driving.
Complicated motion shots are often used for effect or transition rather than to capture lines of dialogue, so depending on your situation you may not even need to worry about noise!
Of course, if you're wealthy enough to afford a segway... or a steadicam for that matter...
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The Comprehensive Guide to No Budget Filmmaking