I have Mitsubishi Fuso FE-649. Just recently, when i accelerate up hills or when cruising down the freeway and accelerate over 65 mph I get this loud hissing sound like an air pressure relief valve has popped, as soon as i let off the accelerator it stops. I looked at the engine for obvious signs, the only thing i can figure is there's a plug at the back of the engine block the you can pull out by hand that is worn out? any help is appreciated.

Thanks, Ray W