High pitch whistle when starting '06 F350 and now check engine light??

[DieselMom]

For the past 2 weeks I have been hearing a high pitch whistle about 2 seconds after I start the engine and it only lasts for like 2 seconds. It doesn't sound like a squeaky belt, but I am no mechanic. I live in So Cal so its not like its cold outside when I start it up. It doesn't feel like the truck is running any different. Then about 3 days ago my check engine light came on. Any thoughts on what the problem might be?? The check engine light had come on about a month ago when it was raining really hard but went off the next day and it hasn't rained since so I figured it was just moisture had got to a sensor or something.
I just bought the truck about 2 months ago with 115,000 miles on it.
Thanks