Cfunk

I have a 2011 ford F150 XLT 5.0L Crew Cab 4X4 and I starting noticing a weird groan/humming/vibration noise under the hood while the truck idles. Seem to be getting worse.After two days at the dealership they say they think the noise comes from my timing belt tensioner.......? From what i've seen from research bad tensioners make a ticking noise constantly..... my noise is not a ticking or knocking and it comes and goes....??? It runs fine...????????????