Sometimes replacing the belt with a new, much tighter one can upset the brushroll until the new belt stretches a little. Most endcaps unscrew with a bit of effort, just, some can be very tight. Silicone spray isnt much good for brushroll bearings, they need to be cleaned with the endcaps removed and then properly greased with some spray grease, to ensure that the grease penetrates inside the ball race. If there is play in the bearings then this will usually create some noise, the worse the wear, the worse the noise. The brushroll end caps need to fit snugly into the chassis recesses, any play here will cause noise as well - some end caps only fit into the chassis in one direction. did you replace the brushroll into the chassis in exacly the same way as it was removed?
Remove the brushroll, and holding both endcaps, spin the brushroll round, if its difficult to turn, or turns anything other than quietly and freely, then the bearings are either worn, or dirt and hair is trapped between the bearings and the endcaps, and the endcaps must be removed to clean out the bearing housings.