Air hissing only under load accelerating seems air compressor and boost related

is on mine anyway. I was having a slightly similar issue with mine dumping air. Replaced pop off and governor and fixed it right up.

 

To me anyway it doesn't exactly make sense that it only hisses when the compressor is unloaded. But still I think I would start with the pop off valve. It should be threaded in the side of your compressor. Then the dryer purge valve and maybe a new governor. Cheap and easy parts. Aside from that maybe the unloader valve, I dunno

 

If you have a Bendix AD-9 dryer it probably has the wrong purge valve in it.
There are a few different ones. the article below explains the differences.

http://www.bendixvrc.com/itemDisplay.asp?documentID=5016

 

Try private messaging Heavyd. He's a long-time member and a HD mechanic. A pop-off/pressure relief valve will exhaust air only when the pressure exceeds a set limit. Usually it's a very loud, continuous burst of air which lasts only a few seconds. What you're describing sounds more like a boost leak, as the sound occurs only when under hard acceleration. Again, message Heavyd. He will have an answer for you.

 

I’m guessing it’s the air intake hose. It’s hard to find t because it’s broken at the compressor, on the backside. It’s the 1” silicone hose from intake manifold to the rear of head on compressor. Look right at the connection to compressor. It’s a tight spot, but doable, without removing anything. No harm done. Just lost boost, and compressor ran on unfiltered air a bit. The intake hose not only is for filtered air, but also serves to turbocharge the compressor.

 

If the intake hose had a leak wouldn't it make noise all the time? Not just when the compressor is unloaded?

 

I guess it would, thinking it’s more noticeable when it’s unloaded. It sounds like the same thing that happened to me once. I didn’t pay much attention to whether compressor was running or not. It’s impossible to see the broken hose, it was broken on the backside, near engine block. Gets hot and fails there. Very common on Detroit’s. Hose needs to be plumbed into an easier spot, or use a stronger hose maybe. But factory design lasts for years. I just replaced the whole hose. Tension clamp makes it hard to access, location makes hard to find/diagnose. It’s such a loud hissing noise, too loud to be just a cracked cac or other minor leak. I may be wrong, but sounds like what’s wrong.