On a 603 engine there shouldn't be any 'slip' of the Intake gasket with all the bolts that locate the manifold. I would strongly recommend using a new gasket as the old ones harden and leak quite easily. Mine was leaking slightly around the #2 intake runner. Make sure the face of the manifold is straight and even and that there is no crud or gasket material on the seating surface. Be sure to use the correct tightening torque when installing, it is easy to strip the aluminum threads in the head
Use one bolt at each end to locate the gasket onto the manifold as you position it on the head. Use cotton thread or fishing line to keep in place if neccessary while you start the bolt threads. Remove the thread/line before you tighten down the manifold. Run all the bolts into place before finally torquing them down.
If there is whistling around the intake then it is probably your old gasket.
You might want to use a new gasket on the cross-over pipe as well.
I don't think you can remove the manifold without un-clipping all the fuel lines - at least not without a lot of extra work. Buy some new plastic clips from the dealer in case you break any during disassembly.
Good Luck
