Sound like you may have abreak in the cable as it enters the dyson (Common Fault) you can get to the cable by removing the switch you require a sharp small screwdriver and gently pus it in to the flat bit oo the switch (nearest the wand) and this will release the snaplock and using a pair of pliers remove the switch button. you will need a tork screwdriver to remove the switch train this will allow you to get a t the cable.
if the motor was varying in speed and noise this sounds like a new motor ?
hope this helps
Tickityboo (the Dyson Man), June 2011
