Shine light towards that area, gun at ready. Don't go in until you're confident enough that whatever's inside can't break out.
Optionally shoot a round into the door/wall, if you don't think anything else will hear you and come running.
hmm maybe just go in with MP5 at ready and start looting the crates. It's a bet, though, since a partial mag of MP5 rounds may not necessarily take care of the monster thing