Working on an 89 XJ with 274k miles on it with the 4.0. Starts hard and there is a hissing/sucking noise. Measured the vacuum and it's around 16-17 inHg. What is normal? When revving it up, it goes into the normal range. What should I be looking at? I've never diagnosed vacuum leaks before. Thanks much!

Check to be sure the intake manifold bolts are snug.