Strange sound coming from intake manifold.

It's started a few days ago but wouldn't do it all the time. Now there is a constant "humming" "whistling" sound coming from the intake manifold (sometimes it's louder than other times). I had my mechanic clean the fuel system 3 weeks ago and now when I start it it wants to rev around 1,500 rpm till it's warmed up, used to idle around 1,000 before fuel system cleaning (not sure if this has anything to do with it). Please help. Thanks

 

Let me add,
I got the fuel system clean because I was getting horrible gas milage. I was using BP or Shell gas. My father said that he was getting good results from Sunoco so I started using that. I did not see any results from the fuel system clean (3 weeks ago) but since I started using Sunoco gas I am getting around 23 + miles extra per tank.
After I posted this msg (40 min ago) I got back in my truck, started it and the sound was still there. I drove 4 miles, filled up my tank again and now the sound is gone... For now but I'm sure it will be back. Any help on this issue would be much appreciated.