Actually, you're bang on on the noise thing. Bear Scout's tail was stepped on right before Checkers started screaming. Checkers, Bear Scout and our other cat Elise (which she's also screaming at) were all in the same area. I'll try the recommendation, thank you so much!
P.S. We are trying Feliway, but it hasn't had a noticeable effect yet. We also tried Rescue Remedy, which also hasn't done much to ease the situation.