It could be that residue from the old belts is sitting at the motor pulley or the idler wheels. This is very easy to clean using cotton buds and tapehead cleaner (isopropanol or carburettor alcohol - it's all the same).
When you have the belts dismounted, try rotating the motor pulley by hand. It should run quietly, no rumble. If the motor rumbles or clicks when rotated it will need to be replaced. Not too hard but requires a little bit of soldering and a couple of screwdrivers including a tiny watchmakers with a flat blade for the motor pulley grub screw. But check for belt residue first.
Martin