I noticed when I changed to alloy wheels my front started pulsing immediately. I installed my perfectly good front disc from the wire wheel onto the alloy. It wasn't warped. I think there is enough slop the way the disc mounts so that the grooves in the disc and pads don't line up anymore. Its gotten better the last couple of thousand miles. If I ever have to remove disc for any reason, I'll go back with new pads.
