Its coming from the back of the engine, Im guessing the rear exhaust manifold. My mechanic replaced the rear head about a year ago. I called another mechanic that deals with nissans more then the first mechanic, and they said that it could be something as simple as the gasket, or he has run across the bolts that hold the header to the block breaking. What is more likely? The car just did it after a long freeway drive.

 

I had a vacuum leak in that area recently. Sounded just like an exhaust leak to me - kind of a pop pop pop at idle. I was sure it was exhaust because a vacuum leak never even crossed my mind. Sounded like it was coming from the rear header, like maybe a broken bolt or a cracked header. Took me about three attempts under the car to find it because the rear manifold is so hard to see. Finally found a vacuum fitting that had backed out.

 

where was this vacuum fitting?

 

The actual connection cannot be seen from the top, however if you know where it is it can be reached from the top. It's a small diameter, (about 1/4") metal line that connects to the block about 6" to 8" below the throttle body. You can easily see the metal line from the top, just not the connection where it meets the block, that's why it took me so long to find. It has a flange type screw in connector that surrounds the line.

 

that connects to the back of the block?

 

Actually into the left rear side of the block. The line I'm talking about is just a little above and rear (toward the firewall) of the starter. That area is crammed with lines, it's the smallest metal one.