1997 Tacoma V-6 making a sucking sound...vacuum?

1997 V-6 making a sucking sound coming from the rear part of the engine on the top. I can't find any signs of where it is coming from. It just started today after I fired it up and drove it around and noticed the check engine light was on...duh. It ran fine but when I stopped at a light I heard it and came home. Any ideas what this could be? It seems like since I had the valve cover gasket replaced...I've had issues...Thanks for any ideas on how to deal with this. I'm not a mechanic just know how to turn a wrench. I'm willing to give it a try, Thanks again.

If you can't see anything obvious stop by local advance auto, auto zone, or O'Reily's and get you're check engine light scanned. That could give you an idea where to concentrate you're efforts.