If you're IKEA bed suffers from a rattling sound, it may simply be the galvanized adjustable cross braces rattling together. This shows a very simple fix for this problem using only a screwdriver and some scotch tape.
Step 1: Remove Mattress and Slats
Take off the mattress and slats to make it easy to get at the cross braces. At this point, you may choose to either remove all 4x cross braces or simply remove one at a time.
Step 2: Mark Positions
You'll want to mark the positions of the two pieces to keep things straight for the next step: taping.
Step 3: Tape the Smaller Rod
To minimize the gaps between the two pieces, apply a single strip of tape to the face of the smaller rod, leaving a gap where the tightening screw will be.
Start by applying a smaller strip to the end being inserted. Try to make sure that there are no creases in the tape which would potentially make the thickness too great. After smoothly folding the tape over the sides, test the tape thickness to make sure it is still able to be inserted without much effort. If there is little or no resistance, you may want to try a slightly thicker tape.
Next apply tape to the next portion by cutting a strip of tape which is longer than needed. Apply the portion closest to the set screw, then use your finger to adhere the tape along the length to minimize creasing. If a crease occurs, pull the tape back up before the crease and try again. Fold the edges over and test that it can still be inserted to the larger rod.
Step 4: Check for Slop
With the two pieces mated together, insert the set screw without fully tightening down. Test for rattling by tapping back and forth by where the pieces overlap. If there's too much rattling, add strips of tape one at a time until the rattling is minimized and the pieces can still slide together. The rattling will be further reduced when the set screw is tightened down.
Step 5: Squaring the Bed
When installing the cross braces, tighten down the screws to the frame and leave the set screws loose.
To square the frame properly, measure the diagonals and adjust them until they match as closely as possible. Tighten the set screws and re-measure to verify that nothing moved while tightening.
Step 6: Strum Those Braces!
Now that everything is tight, you should be able to strum each brace without hearing any rattling. If you still hear rattling, then it's likely from the galvanized support that runs along each side.
If so, simply add some layers of tape between the two surfaces by loosening the single screw to get between them.
And now you're done! Re-assemble your bed and take a nap :)