GMC Repair: duramax engine noise, oil filler cap, tensioner pulley

Question
I have a 2005 GMC 2500 pick up with a 6.6 duramax. When I remove the oil filler cap while it is running you can hear a rattle'n noise coming out off the filler spout.You can't hear anything while the cap is in place.A GM mechanic said it was the 2 little pulley's and the tensioner pulley on the serpentine belt.I removed the belt and started it and the noise was still there,so it can't be the pulley's.Thanks for your help

Answer
How long have you had the vehicle? Did you buy it used? If it were the pulleys you would hear the noise without pulling the filler cap. I am not an "expert" pertaining to diesel engines, however, I have been around them for many years. I have driven over the road for a couple of years. What you are hearing is the valves. I would take it to a mechanic to have the valves adjusted. Diesel engines must sit for 24 hours before the valves can be adjusted.