Hello all.

Recently purchased a 2000 F150 5.4l. Its funny all the little things you notice AFTER you purchase a vehicle. Any ways...
When I take my foot off the gas (at any speed or temp) and coast it sounds like an air leak or hissing coming from behind the dash on the driver's side. It only occurs when coasting and when I get back on the gas it goes away. Just wondering if anyone has any clues on where the sound is coming from and what it may be.

Thanks in Advance!

 

could be a vacume leak on the temp control hoses going to the temp ****.

 

What mods are on the truck?

Sometimes when a FIPK is installed it is not done correctly and you will get a coast down (Negative pressure) noise.