My 99 Frontier 4X4 has developed a ticking in the drivetrain. It is synchronous with the car speed and sometimes changes when brakes are applied (sometimes brakes make it stop, sometimes they make it start??). At first I thought it was a pebble stuck in the brake drum, but, no, brakes are clear. There are no foreign objects in the drive shaft or U-joints or center bearing. I'm thinking the worst (of course) and wondering if there could be a chipped tooth in the planetary gears in the differential. Would that cause a ticking? Is it possible that would be intermittent? If yes, any idea how much it will cost to rebuild the differential? Thanks.