S40 Revving and smoking

[linzrs711]

Hi all, so I am NOT a car person. Today, my Volvo broke down and I was hoping I could get honest opinions on this site because I live in a new area where I don't know many Volvo mechanics.
For the past couple days, I noticed that my car would "rev" while in Drive, stopped at a stoplight. The RPM would shoot up and the temp gauge would drop to really cold for a few seconds, then be normal.
(I've always noticed that my car is VERY loud when it's idle. It seems like a fan is making the noise.)

Today my car started revving again and I noticed a smoke coming from the top passenger side of the hood. I quickly pulled into a parking lot and turned the engine off.

Do any of you know what could be wrong? I'd appreciate any help!!