I took my 2008 3500 MegaCab 6.7 liter diesel truck to my local dealer with a problem noise under the hood on the left front of the engine compartment. The noise almost sounds like a relay chattering extremely fast. The noise at this time is not affecting the performance of the engine, just annoying and wanting to get it fixed before it turns into something bigger. The best way to explain the noise other than a fast relay chatter would be to take a poker card and run it over a bmx bike wheel (The noise we wanted when we were kids riding our bmx bikes); however this is a very very faint sound just under the hood on my truck. It usually happens when I am driving between 20 to 30 mph. Any advice or help will be greatly appreciated.

 

Mine is doing the same thing. Did you happen to ever figure out what it was?

 

Leak detection pump? I know on the gas rigs there's an evap leak detection pump in the left front that can make some noise.