DIAGNOSIS That's almost certainly the internal pinion gear on the motor shaft wearing out or losing its teeth. It's the main gear that drives the wobble plate, and it's a common wear point when the saw is used hard, especially with dull blades or binding.
FIX You'll need to open up the gear housing. First, remove the battery. Use a Phillips head screwdriver to take out the four long screws on the saw's body (two near the blade clamp, two near the handle). Carefully separate the two halves. Inside, you'll see the motor and the black plastic gear case. Remove the three screws holding that gear case on. The pinion gear is the small metal gear pressed onto the end of the motor shaft. If it's chewed up, you need a replacement gear set (often part of a kit with the wobble plate). Press the new one on, reassemble in reverse order, and make sure the halves snap together cleanly.
IF THAT DIDN'T WORK If the noise is more of a high-pitched whine or screech, the bearings in the gear housing might be shot. You'd need to replace the entire gear housing assembly, which includes the bearings and output shaft.
HEADS UP When you reassemble, don't over-tighten the long body screws. Snug is fine. Overtightening can crack the plastic housing or warp it, causing the internal parts to bind.