I'm thinking doofalator valve. In all seriousness though, it probably is just your belt slipping until it heats up (kinda like tires) It doesn't really contract, it's just not as soft and therefore looses grip. Get a new belt. I also had a Nissan pickup that would make a howling noise for a second or two, and then the first couple of times I shifted until it warmed up. It was the throwout bearing, and it wasn't bad, it just howled on cold mornings, did it for years.