Noise in my speakers

Sounds like a problem in the speakers' internal power supply, maybe a faulty capacitor. Speakers need to be looked at by a technician, or replaced with a new set.

This sounds like a bad power supply, or missing earth?

Is it plugged in with a 3.5 millimeter jack? If it is, pull it out of the computer and press you thumb on the tip, if no humming noise comes out, your speakers are probably blown.

However, if a humming noise does come out. Please check to make sure the volume on the computer is turned up/unmuted, the software's volume is turned up/unmuted, your sound card driver is installed (Right-Click "My Computer" -> left click on properties -> Click on the Hardware tab -> click on "Device Manager" -> Click the "+" next to "Sound, video and game controllers" and make sure there is no Red "X"'s or yellow "!"'s.

If you do get a hum, and you do have everything set up right in Windows, try a different pair of speakers or head phones. Your sound card might be fried.

Also, make sure you're plugging the speakers into the correct port on the computer (It's usually a washed out green port"

Select Start > (right click) My Computer > Manage
In the Management window select > Device Manager
Next to Sound, Video and Game Controllers click the + sign
A yellow triangle will signify a problem so if one is present, right-click that device and select Properties
You can then choose to update the driver
For more details check Dell Xps 420 Manual.
Dell Xps 420 Manual

Restar the computer with the headphone plugged and wait the complete start up.. and then put a cd..

Nope in some cases computers have internal speakers but not a very good source four audio some times your monitor will have inboard speakers if that's the case there will be a green conection coming from the back of your monitor that will be hooked up to your computer if and of course your computer will have to have a sound card for any sound to come out other then all that I have stated u will need a set of external pc speakers

Try another speaker set.
If this does not fix it, then consider the sound card if it is removeable, get a new one. If it is internal, turn off the sound and install a sound card.

noise can be generated since dust in the jack connector of your PC, bad sound card or belive or not, the socket. try to unplug another devices and leave alone the x240... compare.

If the sound is working through another device then it seems that the internal speakers are bad. Because if the sound card was bad you wouldn't have any sound whatsoever, same goes for drivers. Probably just some bad speakers. If you have some other external speakers try plugging them into the headphone port and see if that helps.

Connecting headphones to the system will disabled the internal speakers until the headphone jack is disconnected, at which such time the internal speakers should function again.

If you disconnect the headphones and the internal speakers are not re-activated, then there may be damaged to the microswitch in the jack. (Try rebooting after the headphones are disconnected too)