My baby whimpers in his sleep. It freaks me out, so I wake him up and he cries hysterically. Any ideas?

He started it a couple of months ago. He has only done it about 3 times. It is a strange sound and when he wakes, the cry is unlike any other cry he makes. I am certain it is not night terrors, based on what I have read. I think he may be having nightmares but have no idea what would cause this. His environment is stable and he is well loved. I am at a loss.