This morning while warming up the truck, the jake brake was making a hissing noise like an air leak. Is this normal? I turned off the jake and it quit, started again when turned back on. I hadn't heard it before this morning.

 

it is going to make a hiss normaly. . .however my truck when at a cold idle will hiss. . .not sure why,

you could check all the connections and make sure your air supply lines are ok.

ha ha ha. . .air supply

 

It is normal. Mine hisses at an idle all the time.