2003 Nissan frontier 4 cyl, 5 speed making hissing noise when weather is over 85 degrees?

HELP, My 2003 Nissan Frontier 4 Cyl, 5 speed truck is making a hissing noise only when the temprature is over 85 degrees. I took it to a drive shaft and axle shop, they lifted the truck and everything checks out ok with the drive shaft. The noise will not duplicate in the air. To me it sounds like the noise is coming directly from the middle of the truck. This is so wierd. Can anyone help????