I'm having trouble with my 2008 ford F 150 puttering, seems like low to mid RPMs. When I start it up cold it starts idle at about 1500 RPMs, and you can hear a sort of putter type sound on the right side of the engine bay. After about 10 seconds it goes away when the idle drops below a 1000. While I'm driving you can hear it still, it seems to be low to mid RPMs but it's definitely coming out of the right side of the engine bay. I just had a tune-up done at a mechanic shop but hasn't fixed it. Any ideas on what it could be and how to fix it? Thanks!