Funny you ask. After alot of research before I took it to a dealer for $$$$ I just ordered on Friday a little relief valve that goes on the Haldex air dryer my unit uses. I was told on 2008 they used a 175 psi vlave here and it should be a 200 psi. As valve gets worn it starts to blow off easier. This is only what I'm told by a local freightliner dealer. The valve is $17.00 and easy to change (it's as bottom is the air dryer on mine and on the driver side of the dryer at the very bottom.) It's about the size of your thumb. So for that price and my dealers advice I'm changing mine. Were hoping this is what the air popping off is. The part # is HDX KN31529. I should have it by this Wed. Sounds Tumar like it's the same thing. My was releasing air at about 14-20 times steady and varied when it did it but it was not normal. Hope this helps.
(I might of answered this a couple of times - sorry)
Pennie / Ron
2015 Sanibel 3901