It's the steering stops. When at full steering lock, there's a plate on the (a arm?) and an adjustable bolt stop to limit the steering turn on the spindle(?) that move in relationship to each other as the suspension cycles in the bump. When new, there's a rubber boot on the bolt end, and you're supposed to grease the end where it rides on the stop to eliminate the pop sound. You can either grease the bolt head or the plate stop to eliminate/lessen the sound, or just live with it. Use waterproof grease...

I've never tried to find a new boot, but the dealer may be able to get one.