Our 1990 745 GL has developed an intermittently uneven idle, like a cylinder is not firing right or something. When I looked under the hood with the engine running, I noted a pretty loud sucking sound. The sound was hard to locate, and the location of the source seemed to vary depending on from where I was listening, but seemed to be coming from around the front of the intake manifold (when listening from the passenger side of the engine compartment), somewhere near there, but back a bit further (when listening from the front of the car), and generally the same place, but further over, near the head/valve cover (when listening from the driver side of the car). I did check the oil filler cap to see if there was a pressure problem that was either venting or sucking through its seals, but that was not it.

Am going to apply the propane test to the intake manifold this afternoon, as I don't want to pull the gasket without just cause. But the gasket does not look like factory Volvo, and there is a bit of it hanging below the very front stud that holds the manifold on.

While this all makes it look very much like the intake manifold gasket is bad, wouldn't that make for a different symptom than an idle that sounds clearly like (am picking a number here at random, don't know which cylinder it is): one-TWO-three-four, or the like? A leaky intake manifold gasket would be more irregular, would it not?
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1955 Human, Scott; 1991 745 Turbo, Brunhilde; 1990 745GL, Snuggle Bunny