Im in the process of selling my truck but during a test drive, a guy noticed that when you get around 2200 rpm the truck has a strange brief whistle sound. I've had the truck for about 6 months and it has always made the sound but it didn't seem to affect the driveability so I never worried about it. Any ideas on what it could be?

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Problem solved. Found lose clamp going to the intercooler.

 

Make sure you check all clamps as well as checking the boost tube boots, for leaks.