Only posting this to help anyone looking for a particular search term. I had the infamous buzz, rattle, grind, grating sound coming from the left front under the hood. I bought the truck in Nov. 2012 with 83k. It has 88k on it now. Pulled out of the drive last week and this buzzing noise started up. It got louder and faster as I pressed the gas pedal. Didn't do it at idle or revving up in park. Also stopped when I let off the gas and coasted. I scoured the forum looking for somebody having problems with a buzzing noise but only found the aforementioned grinding, rattling description. Lots of reading later and thanks to one poster I decided to test a theory by putting the truck in 4wd to see if it still made the noise. It did not. BAM!, something to do with the vacuum in the 4WD system. I decided to take the chance on it being the solenoid since that seemed to be more likely than the IWE having a leak. Ran to Advanced Auto and picked up a Dorman replacement for $29.99. Slapped it on in 10 minutes and presto, no more buzzing noise. Mine was the original without the plastic hood. New one fixes the original design flaw by shrouding the solenoid so water can't pour right onto it. I suggest that anyone with the original on their 06 spend the money and the 10 minutes to replace with an updated model. Could save you money down the road. This is the ultimate beginner shade tree mechanic fix so go for it. There are other posts on here with pictures as well as some Youtube vids. Solenoid is right behind the battery and has two nuts, two hoses and one connector. Piece of cake.

 

Moved to 04'-08' section.