there are a number of threads on here on the same subject. just use the search button block on the top right of the header page. I have the same problem on my M 931A2 so far ive changed two Emergency brake microswitchs at the primary and secondary tanks , leaking . The PPV valve, pressurization protection Valve leaking, the low range selector valve at the transfer case , leaking, The AIR DRYER ASSEMBLY, bleed valve wouldn't close, Might mention I put on an MRAP dryer, NO MORE MOUISTURE in the system, works really good easy install. Emergency/air spring brake valve at brake handle. and the brake valve at the front mount , still have noise, but fewer leaks , forgot shutterstat air leak, wet tank check valve is next biggest problem was no one regularly drained the air takes, the valves are mostly aluminum and all corroded If you find the cause please post , its a common problem