I went to go take a look at a 05 XJR yesterday and there is a hissing noise coming from underneath the dash ......I believe it may be a vacuum leak from the brake booster or some type of vacuum hose coming from underneath the dash of a 2004 XJR. If I apply the brakes the hissing noise stops I’m assuming quite possibly a vacuum line or a check valve of some sort? Any direction is greatly appreciate it.

 

Hi mine is 2003 4,2 nc lhd and does exactly the same once you release the brake it does that sound . No clue so far what it could be

 

I would think this has to be the brake booster leaking. The x350 has no vacuum operated devices in the dash (HVAC is all electrically actuated, parking brake is electronic). All the check valves are internal to the brake booster. I would confirm this leak and change the booster asap, you don't want to be surprised one day when you have no power brake assist.

 

Thanks everyone brake booster was the culprit, replaced and all is good now!