I have a 2008 F150 with 23,000 miles. At about 20,000 miles my truck started making a humming noise when I accelerate. It does not make this noise all the time but when it does it can be quite loud. It sounds like it might be a problem with a fan or something, and when i took it to the ford dealership they replaced a fan clutch and said that is what they believed was the problem. Problem is, it started making the same noise again. Any suggestions would be much appreciated as to how i might go about fixing this issue. Thanks.