The cupcake can make gentle humming noises or raucous throbbing…depending not only on opinion, but also on how well tuned your machine is. Here are some hints on how to reduce noise, collected from the wisdom of the net.
Why reduce noise at all?
Well, obviously, so your neighbors are not bothered, and you don't go deaf. Or maybe you just want to listen to music at the same time you build something.
Not so obviously, because excess vibration can reduce build accuracy, rattle the machine apart, increase wear, and require you to tighten screws more often.
Also, a well tuned machine will make certain noises, and if something starts to go wrong, a well tuned machine attendant will hear the difference before it is visible, and maybe fix it before something catastrophic happens or before a build is aborted. If the volume of the normal noises is reduced, the abnormal noises will be noticed sooner.
Supporting rods
The supporting rods can cause more noise than just about any other part if they are allowed to rattle in their mounts.
If your X support rods are bit longer than the makerbot, then you can screw the rod cover plates down hard on them and possibly prevent them from rattling. If, however, the X rods are short enough to leave a gap between the end of the rod and the rod covers, it would help to find something elastic to put between the rods and the cover, on both ends (all four plates). Ideally, enough material should be added to more than fill the gap, so there is a bit of pressure on the rod.
Suggested materials include several layers of bicycle tire innertube cut with a hole punch, rubber band pieces, or a piece of folded up paper, cork, rubber from a key chain fob.
Similarly, the Y axis support rods should not be loose. The X cap plates are specifically designed to not lie completely flat against the X front and back, and to wedge the Y axis rods in place, so they are under a bit of pressure and do not rattle.
Also, it is very important to oil all four support rods. A significant amount of noise can come from under oiled rods — if you notice your machine getting louder over time, it's probably time to add a drop of oil to each rod.
Stepper motors
Higher current on the stepper motors causes them to move faster to the next step and with greater force. As a consequence, they will be much louder with higher current. Careful tuning of the current on the stepper motors will significantly reduce both noise from the motors and heat generated by the motors.
However, reducing the current too much will cause the motors to skip steps when they can't produce enough force to move the platform. After reducing the current on the steppers, use the control panel to move each adjusted motor a bit so it is active with the LED's on. Then try to move that axis. The belts should not slip and stepper motor should not turn. If the stepper turns when you apply a small force, the current needs to be raised back up a bit.
Turning the potentiometer on the control board clockwise raises the current, turning it counterclockwise lowers it.
Belts
Belt tension has a large effect on noise. If a belt is overtightened slightly, the belt will vibrate during motion causing increased audio noise. If you really over tighten it, you will cause increased stress on the motors and possibly increased friction on that axis's motion. If you under tighten a timing belt, it will slip.
Belts should be a little loose, but not so loose that the teeth slip.
Also, a torsional spring may also help improve the tension on a belt and reduce noise.
http://www.flickr.com/photos/24375810@N06/3879949941/
http://www.mcmaster.com/#catalog/115/1206/=588967
http://forums.reprap.org/read.php?1,38681
A belt tensioner object has been designed and posted to thingiverse.
Pulleys
Actually, pulleys can generate a lot of noise too. The default _one_ bearing per pulley makes the pulley wobble when rotating under load. Add one more bearing to each pulley if you have any to spare, especially the X and Y axis pulleys - juniortan.operator204
Build platform and stage
If your platform is not sitting flat on the Y stage, it might rattle against it. Make sure all the tabs on the Y stage are trimmed flat or slightly below the flat surface so that the platform is level and flat.
Also, if you switch to a heated build platform using an aluminum base, the base will rattle against the Y stage screws. A temperature resistant rubber mat between the platform and the stage might help.
Also, the panels that make the build stage may vibrate. Tightening the screws might help. A rubber band around loose edges where the endstops could mount might help.
Feet
In a stock cupcake, all that vibration from the build is transmitted through the wood to whatever surface is under the cupcake. Anti-vibration feet, rubber feet, foam feet, or a foam mat or pillow helps.
A design for printable clip-on feet can be found here: http://www.thingiverse.com/thing:1579
Nuts and bolts
Nuts and bolts on the cupcake should not rattle. If they do, they probably need tightening before they fall out. As your machine ages, you may need to periodically tighten the screws. The Y stage screws probably get more vibration than any others, and should be checked more often.