Suddenly this loud hissing noise is coming from the dash or under the dash . It wasn't there one day and the next day it started happening. It is not there while accelerating but only in idle and when it starts to decelerate after putting on the brake. Have lived with it awhile but it's driving me crazy. Could it be a vacuum something or another. I have looked under the hood but don't see anything that has come unhooked. I am a 62 year old woman so I hope that I have explained this well enough. Thanks for any help!

 

You have a vacuum leak.

If it is a Ford truck you're talking about it could be the brake booster or the vacuum controlled heater switch or flaps.

Give some more information about the vehicle and where the hissing is coming from.